Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Copyright © 2009 by The McGraw-Hill Companies, Inc. All rights reserved. Except as permitted under the United States Copyright Act, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without prior permission of the publisher. Send all inquiries to: Macmillan/McGraw-Hill 8787 Orion Place Columbus, OH 43240-4027 ISBN: 978-0-02-288023-1 MHID: 0-02-288023-2 Printed in the United States of America. 1 2 3 4 5 6 7 8 9 10 066 13 12 11 10 09 08 Lesson 1 • Body Systems . . . . . . . . . . . . . . . . . . . . . . 2 Explore Activity: How do body parts work together? . . . . .3 Careers: Careers in Medicine . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Lesson 2 • Bones, Muscles, and Skin . . . . . . . . . . . . 14 Explore Activity: How do muscles and bones work together? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Lesson 3 • Circulatory and Respiratory Systems . 24 Explore Activity: How does air enter and leave the lungs? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25 History of Science: Heart Surgery Pioneers . . . . . . . . . . . . . 34 Lesson 4 • Digestive System . . . . . . . . . . . . . . . . . . 36 Explore Activity: Why is the small intestine full of folds? . .37 Lesson 5 • Nervous System . . . . . . . . . . . . . . . . . . .44 Explore Activity: Can reaction time change? . . . . . . . . . . . 45 Animal Bodies: Wide Nerves for Giant Squids . . . . . . . . . . 54 Lesson 6 • Excretory System . . . . . . . . . . . . . . . . . 56 Explore Activity: How do your kidneys filter out waste? . .57 Lesson 7 • Endocrine and Reproductive Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64 Explore Activity: How do emotions affect the body? . . . . 65 Lesson 8 • The Human Life Cycle . . . . . . . . . . . . . . 72 Explore Activity: How do boys and girls grow differently? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73 Science, Technology, and Society: At the Hospital . . . . . . 80 Lesson 9 • The Body’s Defenses . . . . . . . . . . . . . . . 82 Explore Activity: How quickly can germs spread? . . . . . . . .83 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 iii Lesson 1 • Body Systems . . . . . . . . . . . . . . . . . . . . . . 2 Explore Activity: How do body parts work together? . . . . .3 Careers: Careers in Medicine . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Lesson 2 • Bones, Muscles, and Skin . . . . . . . . . . . 14 Explore Activity: How do muscles and bones work together? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Lesson 3 • Circulatory and Respiratory Systems . 24 Explore Activity: How does air enter and leave the lungs? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25 History of Science: Heart Surgery Pioneers . . . . . . . . . . . . . 34 Lesson 4 • Digestive System . . . . . . . . . . . . . . . . . . 36 Explore Activity: Why is the small intestine full of folds? . .37 Lesson 5 • Nervous System . . . . . . . . . . . . . . . . . . .44 Explore Activity: Can reaction time change? . . . . . . . . . . . 45 Animal Bodies: Wide Nerves for Giant Squids . . . . . . . . . . 54 Lesson 6 • Excretory System . . . . . . . . . . . . . . . . . 56 Explore Activity: How do your kidneys filter out waste? . .57 Lesson 7 • Endocrine and Reproductive Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64 Explore Activity: How do emotions affect the body? . . . . 65 Lesson 8 • The Human Life Cycle . . . . . . . . . . . . . . 72 Explore Activity: How do boys and girls grow differently? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73 Science, Technology, and Society: At the Hospital . . . . . . 80 Lesson 9 • The Body’s Defenses . . . . . . . . . . . . . . . 82 Explore Activity: How quickly can germs spread? . . . . . . . .83 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 iii Lesson 1 Body Systems Pulling on a string will control a kite. How do you control your arms, legs, and other body parts? Do you think your body uses anything similar to kite strings? 2 ENGAGE Materials How do body parts work together? Purpose Explore how body parts and body systems can work together, even when doing very simple tasks. What to Do Work with a partner. Ask your partner to think of instructions for you to use your muscles, such as “Hop on one foot” or “Make circles with your arms.” • stopwatch or timer • pencil • paper Experiment Have your partner call out the instructions. Follow them as best you can. Have your partner time you for 1 minute. Tell your partner how you feel as you use your muscles. Stop after 1 minute or when you feel tired. Step Communicate List the different body parts that you used during step 2. Include the parts that you used for hearing and understanding the activity. Also discuss any changes you observed in your body and the parts that the changes affected. Record your responses. Switch roles with your partner, and repeat steps 2 and 3. Draw Conclusions Classify For each body part you listed in step 3, describe the job it performed. Classify the parts according to their jobs. Infer Could you keep moving your body all day long? Why or why not? Predict What would happen to the human body if its body parts did not communicate with one another? Explore More Choose a body part that you can observe easily, such as the shoulder, wrist, ankle, or foot. Observe and record the ways you use this part throughout the day. 3 EXPLORE How does the body meet its needs? ▶ Main Idea Organ systems work together to maintain cells and to keep the body healthy and active. ▶ Vocabulary cell, p. 4 glucose, p. 4 organ system, p. 6 hormone, p.6 What is the most important thing you will do today? “Breathe” may not have been the first answer that came to mind, but think about how hard it is to hold your breath for more than a few seconds. Whether or not you pay attention to breathing, it is extremely important. If you were not able to keep breathing, you would survive for only a few minutes. Why must you breathe, and breathe so often? Just what does the human body do with all that air? The answers involve the needs of the tiny parts that make up the body. These parts are your cells. The body is made of trillions of living cells. Like the cells of any animal, each cell of the human body needs to take in certain substances and carry out certain processes to stay alive. Food Every cell needs materials that food provides. The digestive system breaks down foods into these materials, then blood carries them to the body’s cells. Glucose (GLOO•kohs) is a type of sugar that cells use for energy. Many foods are broken down into glucose. Oxygen Cells need oxygen to break down glucose and capture its energy. The lungs supply oxygen to the blood, and then the blood carries it to cells. The body must keep breathing because cells use oxygen very quickly, and they cannot store it. Water Did you know that water makes up most of the human body? As water is used by cells, it must be replaced. This is why you should drink plenty of liquids—such as water, juice, or milk—every day. ◀ Body cells use food and oxygen to meet their need for energy. The body is organized to meet the needs of every cell. 4 EXPLAIN Blood Cells as Carriers food, oxygen, and water blood cells energy carbon dioxide and other wastes ▲ Cells use the energy from food to do different jobs for the body. ▲ Blood provides the food, oxygen, and water that body cells need, and it carries away the wastes. Read a Diagram Why do body cells need to be near a blood supply? Waste Removal As cells break down glucose, they make a waste gas called carbon dioxide. The blood carries carbon dioxide to the lungs, where it is breathed out. Blood also carries away other waste products of cells. Muscle cells use energy to move. Muscle cells cannot move from place to place in the body, but they can get longer or shorter. By working together, many muscle cells pull on bones that are joined to them, moving arms, legs, and other parts. Communication The body could not function if its trillions of cells each acted on their own. Communication among cells allows the body to carry out coordinated, complex tasks. These tasks include growth and movement. Red blood cells work as carriers. They carry oxygen to and carbon dioxide from cells. White blood cells work as protectors. They fight bacteria and other germs that could harm the body. Jobs for Cells All cells gets energy from food and oxygen. They use the energy to do different jobs. Some cells make special products. For example, certain cells in the stomach make an acid that helps break apart food. Cells in the liver make bile, a thick liquid that helps break up fats in food. Other cells act as messengers. A nerve cell might signal a muscle to tighten or relax. Or it might carry information from the eyes, ears, or other sense organs to the brain. Blood also carries messages from one cell to another. The cells of the body do all these jobs and many others. Quick Check Critical Thinking Why does your body need to breathe? 5 EXPLAIN Organ Systems To meet the needs of its cells, the human body is organized into organ systems. An organ system is a set of body parts, called organs, that work together to complete a task. In humans, organ systems are also called body systems. Many organ systems work all the time; others work only when the need arises. All organ systems work together to keep the body healthy and active. Quick Check Critical Thinking How are different body systems related? Excretory Organs: kidneys and urinary system, end of the digestive tract, lungs, skin Respiratory Organs: mouth and nose, air tubes, lungs Job: breathing, which brings oxygen in and takes carbon dioxide out of the blood Immune Organs: lymph nodes and ducts, spleen, thymus (THIGH•muhs), and other organs Endocrine Organs: glands in the head, neck, and elsewhere Job: making hormones, which are chemical messengers that travel through the blood Reproductive Organs: ovaries in women, testes in men, and other organs Job: reproducing Job: removing wastes from the body 6 EXPLAIN Job: stopping bacteria and other agents of disease Digestive Organs: mouth, esophagus (i•SO•fuh•guhs), stomach, intestines, and many glands Job: digestion, or the breaking down of food into pieces the body can use Skeletal Nervous Organs: brain, spinal cord, nerves, sense organs Job: controlling other body systems; sensing and responding to the environment; thinking and remembering Muscular Circulatory Organs: heart, blood vessels Job: bringing blood to every cell of the body Integumentary Organs: bones and other hard tissues Organs: muscles and tendons Organs: skin, hair, and nails Job: supporting and protecting the body; allowing it to move at joints Job: moving bones and other organs Job: covering and protecting the body 7 EXPLAIN Systems Working Together None of the organ systems of the body could work on its own. Each depends on the other organ systems to do its job. They all work together to perform even simple tasks. For example, your teacher might give you a set of instructions. The instructions are to go to your desk, find a pencil and a piece of paper, and write the name of your first pet. Under the guidance from another part of your brain, your muscular and skeletal systems moved your body in just the right way so you could get to your desk. They also allowed you to grasp a pencil and write with it. In addition, your circulatory and respiratory systems were working to bring glucose and oxygen to all of your body parts so that they could carry out their functions. These five organ systems work together whenever you need to perform a task. Completing this task involves at least Often a sixth system joins them. Your body five organ systems. Your nervous system needs different amounts of glucose signaled the nerves in your eyes and parts depending on whether you are exercising, of your brain to read the instructions, to resting, or sleeping. Hormones produced by remember where your desk is located, and the endocrine (EN•duh•kruhn) system help to search for a pencil and paper. Meanwhile manage glucose levels to meet the body’s your brain remembered the name of your changing energy needs. Your endocrine pet, as well as the spelling of that name. system responds to your activity level. The endocrine system helps other systems work together to meet the body’s needs.▼ 8 EXPLAIN Keeping Healthy When the body is healthy, every body system works properly. But if a part breaks in one system, other systems could be affected. Body systems communicate their needs through the nervous system. Feelings of hunger, thirst, and tiredness all tell the brain to take action in a way that helps the body. Even pain is useful. Pain signals the brain that a body part is injured and needs help. In mild cases, such as a stubbed toe, resting the body part is all that is needed. In cases of severe pain that does not improve, help from a health-care expert might be necessary. Ways to stay healthy include exercising and eating a balanced diet. Exercising helps maintain the body at a healthy weight. It strengthens bones and muscles, and it improves sleep. Exercising can even help the mind! It increases blood flow to the brain and stimulates the production of hormones that improve mood. Activities that increase the heart rate, such as jogging, may help people feel happier and remember things better. Ad Review What to Do People buy many products to improve their health or keep them safe. Find ads for these products in old magazines and newspapers. Classify Present the ads to the class. Group them according to the body systems they help or protect. Evaluate Explain how the ads try to get people to buy the products. Identify ads that you think are effective. Draw Conclusions Infer Why might people buy health and safety products they do not need? Discuss. If you have questions about your health or your body, you can ask your parents, your teacher, or a doctor, nurse, or other health expert. Your body must last a lifetime, so care for it wisely! Daily exercise is an important key to healthy living. ▶ Quick Check Critical Thinking Why must body systems work together? 9 EXPLAIN Studying the Body How have scientists learned so much about organs and organ systems? When you visit a doctor, how can he or she learn what is happening inside you? The answers to these questions involve medical technology—tools for studying and treating the human body. Doctors also depend on science skills, such as observing, inferring, and drawing conclusions. Quick Check Draw Conclusions When might a doctor choose each of the technologies shown here? Medical Technology Microscope Scientists have studied cells from every body organ. By looking through a microscope, they can tell if a cell is normal or damaged. They also can spot invaders, such as germs, as well as diseases, such as cancer. X-ray Skin and other soft body parts let X-rays pass through them. But bones and teeth block X-rays, so they leave shadows on an X-ray picture. To picture the heart or blood vessels, a special dye is injected into the patient. The dye blocks X-rays much like hard body parts do. MRI Unlike X-rays, a technique called MRI can take a detailed picture of any body part. The three letters stand for magnetic resonance imaging. An MRI uses the body’s atoms and molecules to show internal body parts in great detail. The technique uses powerful magnets, a special dye, and a computer. Stethoscope Doctors use stethoscopes to listen to the body’s internal sounds. The heart, lungs, and intestines all make sounds, and each sound is a clue to how well an organ is working. Many doctors would say that the stethoscope is one of the most important tools they use. 10 EXPLAIN Visual Summary The body is made of trillions of cells. Each cell needs food, including glucose for energy. Cells also need oxygen, water, and waste removal. An organ system is a group of organs that work together to complete a task. These tasks include circulating blood, digesting food, and moving the body. Organ systems work closely together. Bones and muscles work together to move the body. Organ systems respond to meet the body’s changing needs. The nervous and endocrine systems act as coordinators and messengers. Writing Link Think, Talk, and Write Main Idea How does the body meet all of its cells’ needs? Vocabulary A chemical messenger called a(n) ______ travels through the blood from one cell to another. Classify List three examples of your body acting automatically. List three examples of your body acting from your thoughts. Critical Thinking Why is rapid communication important in the body? Give an example to support your answer. Test Prep The kidneys, skin, and lungs are each a part of the respiratory system. excretory system. integumentary system. nervous system. A B C D Test Prep Cells of the immune system act as _______ for the body. A carpenters B special-delivery messengers C protectors D clean-up crews Social Studies Link Write an Essay History Report How are the cells of the human body like a single-celled organism? How are they different? Write an essay that compares and contrasts these two types of cells. Research an event or discovery from the history of medicine. Examples include the work of Joseph Lister, Louis Pasteur, Marie Curie, or Charles Drew. Share your report with the class. 11 EVALUATE Careers in Medicine Physician Physicians (fuh•ZI•shuhnz), or medical doctors, are experts on the human body. They are the leaders of the health-care team. Physicians study every body system and the common diseases that affect them. Yet many physicians are specialists, meaning they treat only certain kinds of illnesses. A cardiologist treats heart diseases. A neurologist treats diseases of the nervous system. Some doctors specialize in certain kinds of patients. For example, pediatricians treat children. Most physicians see patients in the hospital, in private offices, or at clinics. Some physicians research cures for serious diseases, such as cancer and AIDS. Many work to bring good health care to communities that need it. To be a physician, a person must usually study for 4 years in college, then for 4 years in medical school. In order to specialize in a certain area, a physician must often have an additional year or two of training. ▲ Communicating with patients is an important part of a physician’s job. 12 EXTEND Careers Physical Therapist When you were a baby, your body learned how to use the muscles in your legs, arms, and other body parts. Even now you are learning new ways to move. You practice these movements when you play sports, study a musical instrument, or learn a dance step. A physical therapist may work with patients for weeks or even months to help them regain lost movement. ▼ However, people sometimes lose the use of muscles and body parts, either partially or totally. The cause could be a broken bone, a genetic disorder, or an injury to the nervous system. Recovering from serious injuries and diseases takes time and effort. In many cases, physical therapists can help. They might lead a patient in exercises that retrain the body to use a non-working part. Or they may help patients find new ways to move their bodies. Physical therapists must earn a degree and a license. Many work in hospitals or doctors’ offices. Others work in offices of their own. Write About It Research How do physical therapists do their job? What equipment do they use? Research physical therapists and create a catalog of the different pieces of equipment they might use. Be sure to include what each piece of equipment is used for. 13 EXTEND Lesson 2 Bones, Muscles, and Skin In ancient times running fast helped people catch food and escape danger. Today people run fast in races. What body parts work together to help you run? 14 ENGAGE Materials How do muscles and bones work together? Form a Hypothesis Muscles work in pairs to move bones. What would happen if one muscle in a pair stopped working? Write your answer as a hypothesis in the form “If one muscle failed, then….” • 2 cardboard strips, each 30 cm long Test Your Hypothesis Make a Model Punch a hole near the end of each • string cardboard strip. Put the brass fastener through the holes to fasten the two strips together. Tape pieces of string to the model and add a paper hand to the end of one strip as shown. • hole punch • brass fastener • tape • paper hand cutout Observe Place the model arm flat on the desk. Hold the end that would attach to the shoulder. Pull the strings one at a time. Observe the motion of the arm. Remove one string, and then pull the string that remains. Describe how the arm’s range of motion has changed. Experiment Move the remaining string to other positions along the arm. Pull the string, and compare the results to those from step 2. Step hold here pull pull Draw Conclusions Infer What do the strings and cardboard represent in your model? Interpret Data Did the results support your hypothesis about muscles? Explain. Predict If one muscle that crossed an elbow stopped working, predict what would happen to the elbow. Explore More Choose a pair of bones or a joint in the body that moves differently than the elbow. Build a model of it. How does your model compare to the elbow model? 15 EXPLORE What are the bones of the body? ▶ Main Idea Systems of bones, muscles, and skin protect, support, and move the body. ▶ Vocabulary cartilage, p. 16 skeletal system, p. 16 joint, p. 18 ligament, p. 18 muscular system, p. 20 dermis, p. 22 epidermis, p. 22 Compare the top of your head to your nose and your ears. The skull is made of hard bone. Parts of the nose and ears are made of a softer material called cartilage (KAHR•tuh•lij). Cartilage is also found at the ends of many bones and in tubes that carry air to the lungs. The adult skeleton has 206 bones. Bones and cartilage make up the skeletal system. This system supports, protects, and gives shape to your body. It also works with the muscular system to move your body. Skull The skull is a set of more than 20 bones, most joined tightly together. The skull surrounds the brain, protecting this delicate organ. Three tiny bones make up the middle ear: the hammer, the anvil, and the stirrup. They have important roles in hearing. Spine The spine runs down the center of the back. It is made of separate bones called vertebrae (VUHR•tuh•bray). The spine surrounds and protects a set of nerves called the spinal cord. It also supports the entire body, allowing it to stand upright. Ribs and Sternum Twelve pairs of ribs are attached to the spine and wrap around the body. Most ribs connect to a flat bone called the sternum. Together they protect the heart, lungs, and other organs of the chest. Collar, Shoulder, and Hip Bones These bones have curved shapes. They support the arms and legs, allowing them to move freely. Arm and Leg Bones The arm and the leg each contain one long bone in the upper half of the limb and two long bones in the lower half. Both the hands and the feet contain many small bones. Quick Check ▲ A helmet acts like an extra skull. Both help protect the brain, a very important organ. 16 EXPLAIN Critical Thinking Why is it useful for the skeletal system to have a wide variety of bones? Bones of the Skeletal System skull mandible (jaw) clavicle (collarbone) humerus ribs radius vertebrae pelvis femur patella tibia Read a Diagram What clues do bones provide about the body parts around them? 17 EXPLAIN Types of Joints Joints The human body can move because of its joints (joynts). Joints are the places where two or more bones meet. The body uses many types of joints, each allowing a different range of motion. Many joints are held together by ligaments (LI•guh•muhnts), which are tough, ropelike tissues. A ball-and-socket joint allows movement backward, forward, and side-to-side. These joints are found in the shoulders and hips. Some joints cannot move. These are called immovable joints. For example, the joints among the many bones that make up an adult’s skull are tightly woven together. The joints between vertebrae move only slightly. This movement allows the back to arch, but not to bend at a sharp angle. Inside these joints are cushionlike discs. They absorb any up-and-down forces on the spine. They allow the body to walk, run, and jump with ease. Free-moving joints allow body parts to bend or twist. Inside each of these joints is a space filled with fluid. A hinge joint allows backward and forward movement, like the hinge of a door. These joints are found in the elbow, knee, fingers, and spine. When a joint is moved beyond its limits, ligaments may tear, twist, or break. This is called a sprain. Sprains often take a long time to heal. ▼ Joints in the arm, shoulder, and neck are all different, but they all allow for movement. A saddle joint allows movement backward, forward, and side-to-side, but limited rotation. A saddle joint connects the thumb to the rest of the hand. 18 EXPLAIN More About Bones Bones are hard and tough, yet still light enough for the body to carry and move. They are also made of living cells. Blood vessels bring blood to and from bones, just as they do for other organs. Nerves travel to bones, too. Most bones are made of several layers. The outside is a thin, hard layer called compact bone. Underneath is a slightly softer layer called spongy bone. The centers of bones are filled with a jellylike tissue called marrow (MER•oh). Marrow can be red or yellow. Red marrow makes most of the body’s blood cells. Bones form from cartilage. Babies are born with a mixture of bone tissue and cartilage in places where adults have only bones. In children cartilage builds up on the ends of bones. This cartilage is gradually replaced by bone tissue over time, making the bones longer and harder. This process continues until just after the teenage years. When the cartilage stops building on the bones, the bones stop growing longer. At this time the body stops growing taller. Bone tissue is constantly being broken down and rebuilt. This process helps heal bones when they break. Quick Check Critical Thinking How is bone tissue more useful in the body than steel or strong plastic would be? Inside a Bone compact bone spongy bone bone marrow ◀ A cast helps a broken bone to heal properly. The two broken ends of the bone must meet exactly. 19 EXPLAIN Muscles Bones and other body parts are attached to organs called muscles. All the muscles make up the muscular system, which moves the body. It also helps keep the body in balance, allowing it to stand up straight and hold other positions. There are three types of muscle: skeletal, smooth, and cardiac. Although the brain controls every muscle, only the skeletal muscles’ movements are voluntary. This means that the brain decides when and how to move them. Other muscles’ movements are involuntary. The brain controls them automatically. Most skeletal muscle attaches to bones with tendons (TEN•duhnz), which are tough, cordlike tissues. Ligaments hold tendons and bones in place. The fibers that make up skeletal muscle are organized to contract, or shorten, but they only exert force in one direction. They can only pull on bones. They do not push them. For these reasons, most bones are moved by pairs of muscles. When one muscle contracts, the other relaxes. This causes movement. ▲ With practice, muscles can move the body in amazing ways. They also become stronger with exercise. Working muscles use a lot of energy. They depend on the circulatory system to bring glucose and oxygen to them. Muscles also store glucose, which they break down when energy is needed. Muscle Types Skeletal muscle • moves bones • voluntarily controlled • has striped cell structure 20 EXPLAIN Smooth muscle • moves the intestines, stomach, and other organs • automatically controlled • has no stripes in cells Cardiac muscle • pumps the heart • automatically controlled • has striped cell structure that differs from skeletal muscles Working Together A typical pair of skeletal muscles crosses a joint on two sides. For example, the biceps muscle crosses over the elbow, and the triceps muscle crosses below it. When the biceps muscle contracts, the elbow bends to make a smaller angle, bringing the hand toward the shoulder. This motion is called flexing. When the triceps muscle contracts, the angle widens. This motion is called extending. Under the brain’s control, the biceps and triceps work together to flex and extend the elbow. Your muscles and brain learned how to do this when you were a baby. Now when you decide to flex or extend your elbow, the brain sends exactly the right signals to make it happen. triceps muscle contracted Muscle Model Muscle tissue is made of many fibers. The fibers never shorten, so why does the muscle shorten? Form a hypothesis, and then test it with a model. Materials: 3 pencils of equal size What to Do Make a Model Hold the pencils between the fingers of both hands as shown. A partner may help you position them. Slowly move your hands together along the pencils. This models how a muscle contracts. Draw Conclusions Does the model support your hypothesis? Explain. Pairs of muscles work to bend joints all over the body. As a general rule, the largest, thickest muscles move the heaviest body parts, such as the legs and upper arms. Thin, delicate muscles move the fingers and toes. Even the eyes use muscles to move. A set of muscles rotates an eyeball up, down, left, and right. biceps muscle contracted Quick Check Critical Thinking Why must muscles work in pairs to move a bone at a joint? 21 EXPLAIN The Skin What is the body’s largest organ? It’s the organ on the outside—the skin. The skin separates and protects the internal organs and tissues from everything outside. Nerves that end in the skin sense pain, touch, and temperature. Certain skin cells use sunlight to make vitamin D. Oils in the skin keep it soft and flexible. The skin has two layers. The inner layer is the dermis (DUHR•muhs). The dermis contains blood vessels, nerves, glands, and hair follicles. The outer layer is the epidermis (e•puh•DUHR•muhs). Skin is part of the integumentary system. This body system also includes hair, fingernails, and toenails. Both hair and nails are made of a tough material called keratin (KER•uh•tuhn). Nerves and blood do not travel into hair and nails, which is why haircuts and nail filing are not painful. Skin is also part of the excretory system. When the body gets hot, sweat glands in the skin release a salty liquid called sweat. This cools the skin as the water in the sweat evaporates. Skin cells divide rapidly on the inner edge of the epidermis and are pushed to the outside. Eventually the cells die and flake away. The body loses about 35,000 skin cells every minute! Quick Check Critical Thinking Why is blood needed in the dermis but not in the outer cells of the epidermis? ▼ Skin is made of layers of cells. Hair, sweat, and oils all come from the inner layer, the dermis. oil gland epidermis nerves dermis hair follicle blood vessels 22 EXPLAIN sweat gland Visual Summary The skeletal system provides a framework to support and protect the body. It is made of bone and cartilage. Bones meet at joints. Some joints do not move. Others move partially or freely. Think, Talk, and Write Main Idea What are the jobs of muscles, bones, and skin? Vocabulary Tough tissues called ______ hold bones in place. Cause and Effect The brain controls smooth and skeletal muscles in different ways. How does this difference help the body work? Cause Effect Critical Thinking The human arm has The muscular system uses three types of muscles: skeletal, smooth, and cardiac. Skeletal muscles are the only type with voluntary movement. The dermis is the inner layer of the skin and is made of living cells. The epidermis is the outer layer. It sheds dead cells. Writing Link 30 bones, but 27 of them are in the hand and wrist. What does this explain about the way arms and hands work? Test Prep Where in the skeletal system are blood cells made? A cartilage B hair C fluid inside joints D red bone marrow Test Prep Sweat glands, hair follicles, blood vessels, and nerve endings are found in A bones. B muscles. C the dermis. D the epidermis. Math Link Research Report Count Skin Cells Research scoliosis, a condition in which the spine is unusually curved. In your report discuss its causes, effects, and treatments. The body loses 35,000 dead skin cells every minute. Calculate how many skin cells it loses in 1 hour, 1 day, and 1 week. Round each answer to the nearest million cells. 23 EVALUATE Lesson 3 Circulatory and Respiratory Systems Your lungs have millions of these tiny air sacs. What force pulls air into them? What force pushes air out? 24 ENGAGE Materials How does air enter and leave the lungs? Make a Prediction Your body gets the oxygen it needs from air. How is air moved into and out of the lungs? Make a prediction. Test Your Prediction • plastic bottle with its bottom cut off Make a Model Use a rubber band to attach the • drinking straw opening of the small balloon to one end of the straw. The balloon represents the lungs. The straw represents the airways to the lungs. • scissors • small balloon Insert the straw and balloon into the top of the bottle. Seal the hole with modeling clay. • large balloon Cut open the large balloon. Stretch it across the open bottom of the bottle. Seal it with a rubber band or tape. • modeling clay • 2 rubber bands Experiment Pull down slowly on the stretched balloon, then release it. Observe what happens to the model lungs. Record your observations. Step Draw Conclusions Compare Muscles act to move the body. Which part of your model acts like a muscle? What changes does it cause? Infer Using your model as a guide, describe how air moves into and out of the lungs. Explore More How does your chest change size as you breathe? Make a prediction, and design a way to test it. 25 EXPLORE Why must the body breathe? ▶ Main Idea The circulatory and respiratory systems work together to bring oxygen to body cells and to take away carbon dioxide from them. ▶ Vocabulary respiratory system, p. 26 alveoli, p. 27 capillary, p. 27 diaphragm, p. 27 circulatory system, p. 28 artery, p. 28 vein, p. 28 hemoglobin, p. 30 Breathing is important for two reasons. First, breathing takes in oxygen, which cells use to break apart food to get energy. Second, breathing removes carbon dioxide, a waste gas from cells. The body cannot store oxygen or carbon dioxide for long periods of time. This is why you need to breathe continuously, even while you are asleep. Breathing is the job of the respiratory system. The lungs are the main organs of this system. Other organs are the airways that lead to the lungs, including the mouth and the nose. To and From the Lungs To inhale is to breathe in. When you inhale, air is pulled into your lungs. The air travels through your mouth or nose and then down a stiff tube called the trachea (TRAY•kee•uh). The trachea is the main tube in a system of branching air tubes, much like the trunk and branches of a tree. The Respiratory System nose mouth The motion of the diaphragm pulls air through the mouth and nose into the lungs. It then pushes air back out along the same path. ▶ lungs diaphragm 26 EXPLAIN Near the lungs, the trachea divides into two tubes called bronchi (BRONG•kigh). The bronchi branch into smaller tubes that end in tiny air sacs. These sacs are called alveoli (al•VEE•uh•ligh). An adult lung has hundreds of millions of alveoli! All alveoli are surrounded by capillaries (KA•puh•ler•eez). A capillary is a short, narrow blood vessel. In a healthy lung, thin membranes separate alveoli and capillaries. Gases pass easily across these membranes. Oxygen passes from the alveoli into the blood. Carbon dioxide passes from the blood into the alveoli. Carbon dioxide leaves through your mouth and nose when you exhale, or breathe out. As you inhale and exhale over and over, the respiratory system carries out the cycle of gas exchange. The Diaphragm The muscles of the rib cage and the chest force air in and out of the lungs. The largest of these muscles is the diaphragm (DIGH•uh•fram). The diaphragm is the long, flat muscle beneath the lungs. When you inhale, your diaphragm contracts and moves downward. This makes your chest larger and creates a vacuum. Air rushes into your lungs to fill the vacuum. The opposite process happens when you exhale. Your diaphragm moves up into the chest, making your chest smaller. Air is then pushed out of your lungs. Quick Check Critical Thinking What would happen if a person’s diaphragm were injured? Gases exchange between alveoli and the capillaries that surround them. ▶ trachea alveoli bronchi Read a Diagram What path does air follow as you breathe? ▲ A series of air tubes lead through the lungs to the alveoli. 27 EXPLAIN The Circulatory System To circulate is to move in a circle or through a cycle. The circulatory system works to keep blood moving through all parts of the body. Blood circulates during times of exercise and times of rest. It circulates whether the body is standing up straight, lying flat, or hanging upside down! Arteries carry blood away from the heart. The heart is the main organ of the circulatory system. The heart acts like a pump, supplying the power that keeps blood moving. The heart lies in the center of the chest, between the lungs. If you place your hand on the skin above your heart, you can feel it beating. Blood vessels are the other organs of this system. A blood vessel is a tube that carries blood. The three types of blood vessels are arteries (AHR•tuh•reez), veins, and capillaries. A Path for Blood Blood may travel in many paths as it cycles through the body. Yet each path leads into and out of the heart and the lungs. Blood that leaves body cells is low in oxygen and high in carbon dioxide. It gathers in the veins that lead to a heart chamber called the right atrium. Next blood passes to the right ventricle, which pumps it to the lungs. Veins carry blood back to the heart. Key: Oxygen-rich blood Oxygen-poor blood 28 EXPLAIN Capillaries connect arteries and veins. Gases, food, and wastes travel back and forth through the capillary walls. The Heart The heart is a twosided pump. The right side pumps blood to the lungs. The left side pumps blood to the rest of the body. aorta pulmonary artery 3 1 Oxygen-rich blood from the lungs enters the left atrium. Oxygen-poor blood from the body enters the right atrium. 4 2 As the left atrium contracts, oxygenrich blood flows into the left ventricle. The blood is pumped to the rest of the body through the aorta. As the right atrium contracts, oxygen-poor blood flows into the right ventricle. The blood is pumped to the lungs through the pulmonary artery. Inside the lungs, oxygen moves into the blood as carbon dioxide leaves. The oxygen-rich blood then flows back to the heart, this time to the left side. The left ventricle pumps this blood into the aorta (ay•OR•tuh), the body’s widest artery. Several arteries branch off the aorta. Many branches lead to the head and arms. Other branches travel to heart muscles. The lower part of the aorta leads to the legs. Arteries continue branching to reach every body part. As they branch, they get smaller and smaller. Eventually, they become capillaries, which are only one cell thick and very fragile. The oxygen leaves the blood in the capillaries and travels to the cells. At the same time, carbon dioxide and other wastes move from the cells into the blood inside the capillaries. The end of a capillary is joined to a small, narrow vein. Veins are organized much like arteries. A huge number of small veins join to form larger ones, eventually leading back to the heart. Quick Check Critical Thinking How do the two sides of the heart compare? 29 EXPLAIN red blood cell white blood cell serum ▲ Blood travels to all body parts. It carries gases, food, proteins, and clotting agents. Blood What is blood? Blood is a movable body tissue. It is made of many materials, including cells and parts of cells. The liquid part of blood is called serum. Blood does many important jobs, including carrying gases and other materials, clotting, and protecting the body from disease. Carrying Gases As you know, blood carries the gases oxygen and carbon dioxide. Both gases bind to a special compound in red blood cells called hemoglobin (HEE•muh•gloh•buhn). Hemoglobin contains iron and gives these cells their red color. Carrying Food, Proteins, and Wastes Blood serum contains all sorts of things that cells need, as well as waste products of cells. From capillaries, serum leaks out of the blood and moves into the lymph (limpf). 30 EXPLAIN Lymph is a pale fluid that surrounds body cells. It removes cell wastes. Lymph collects in vessels that are much like blood vessels. The vessels join together and lead to lymph nodes, which filter the lymph. Then the lymph is returned to the blood. Clotting When blood is exposed to air, it forms a dry, sticky mass called a clot. A blood clot stops blood from flowing out of a wound. Forming blood clots is the job of blood proteins and platelets (PLAYT•luhts). Platelets are small pieces of blood cells. At the site of a wound, platelets clump together while blood proteins form sticky threads. These combine to form a clot. Fighting Invaders White blood cells fight invaders, such as bacteria and viruses, that could make the body unhealthy. The blood carries many kinds of white blood cells, each of which has an important role. ◀ To measure blood pressure, a cuff squeezes the patient’s arm. Changes in blood sounds indicate the pressure. Tube Pressure Blood Pressure Blood pressure is the force of moving blood against the walls of blood vessels. This force comes from the heart’s pumping. The circulatory system works best under a certain range of blood pressures. If the pressure drops too low, blood will circulate poorly. If pressure rises too high, it could cause a blood vessel to burst or the heart to be damaged. For otherwise healthy people, high blood pressure is a more common problem. As the body ages, arteries can become narrower and stiffer. Smoking tobacco and eating too many fatty foods can also damage arteries. Blood pressure rises as the heart is forced to pump harder. Quick Check Critical Thinking How might fatty foods contribute to high blood pressure? Model the heart and blood pressure by blowing air through openings of different sizes. Materials cardboard tube, waxed paper or aluminum foil, rubber band, scissors What to Do Use Models Use a cardboard tube to model an artery. Your mouth and lungs will model Step the heart. Hold the tube at your mouth and blow air through it. Record the results. Compare Use the rubber band to secure waxed paper or aluminum foil across one end of the tube. Then cut a hole about the size of a quarter. Repeat step 1, and compare the results. Compare Repeat step 1 again with a new piece of paper and a hole the size of a pin prick. Draw Conclusions Interpret Data How can narrow arteries lead to heart disease? 31 EXPLAIN Heartbeats The heart works differently from other muscles. Its different regions contract and relax in a regular cycle. One complete cycle is called a heartbeat. Typically the heart beats from 60 to 100 times a minute. Doctors can use this signal to study the heart. By measuring and recording electrical changes through the skin, doctors can learn how well the heart is working. This measurement is called an electrocardiogram (i•lek•troh•KAHR•dee•uh•gram), or EKG. An EKG is a kind of line graph. The A nerve from the brain affects the rate, spikes and dips of the line repeat in a cycle. or speed, of heartbeats. But the heart would This shows the cycle of the heart beating. continue beating even if that nerve stopped working. An electrical signal within the Quick Check heart controls how it beats. Critical Thinking Why might an EKG be more useful than listening to a heartbeat with a stethoscope? ▼ To obtain an EKG, wire leads are attached to the patient’s skin. Doctors often want patients to exercise during the procedure. This shows how the heart responds to stress. 32 EXPLAIN Visual Summary The circulatory and respiratory systems work together to bring oxygen to body cells and to remove carbon dioxide. Think, Talk, and Write Main Idea By working together, what job do the circulatory and respiratory systems do? Vocabulary Inside the lungs, gases move between capillaries and ______. Sequence Beginning with the right Airways in the lungs end in tiny air sacs called alveoli. Gases pass between alveoli and capillaries. atrium, describe the path that blood takes through the body. First Next Last The heart, arteries, veins, and capillaries move blood through many paths in the body, but all paths lead through the heart and lungs. Blood has many parts, including red blood cells, white blood cells, and blood serum. Blood platelets and proteins help form blood clots. Writing Link Critical Thinking If blood did not form clots, what harm could be caused by even a small cut on the skin? Test Prep What part of a tree is most like the trachea? A trunk B upper branches C small twigs D leaves Test Prep What is the role of hemoglobin? A forming blood clots B carrying glucose C carrying oxygen and carbon dioxide D fighting invaders Art Link Journal Writing Healthy Heart and Lungs Write journal entries from the perspective of a red blood cell as it travels through the circulatory system. Use vocabulary words from the lesson. Research ways to keep your heart and lungs healthy. Present one or more of these ways in a poster. Display posters around the classroom or school. 33 EVALUATE Heart Surgery Pioneers If the heart ever stopped pumping blood, Surgeons also can replace faulty heart the body would quickly die. This is why valves and other parts of the heart. When a heart disease is very dangerous. A faulty new heart or valves are transplanted, the heart is difficult to fix or replace. patient’s body does not recognize the new tissue. It may “reject” the heart by In 1967 Dr. Christian Barnard led the attacking it as though it were a germ. team that performed the first successful heart transplant. The patient only lived for Anti-rejection drugs help the body accept a new heart and heart tissues. a few weeks, but greater successes soon followed. The first permanent artificial If a patient has blocked arteries, heart heart was put into a patient in 1982. surgeons can do bypass surgery. This lets Today heart transplants are quite common. the blood flow around the blockage. A surgical team worked for 9 hours to transplant a new heart into their patient. Dr. Barnard is the tall man at the left of the photograph. ▼ 34 EXTEND History of Science ▲ Using a stethoscope, a doctor can listen to heartbeats. An irregular rhythm can indicate potential health problems. Write About It Draw Conclusions 1. Why would a heart transplant take many hours to perform? 2. Would it be better to replace a patient’s entire heart or only a part of the heart, such as a valve? Why? ▲ Doctors have also experimented with artificial hearts, such as the Jarvik-7 shown here. 35 EXTEND Lesson 4 Digestive System The small intestine is full of folds. Stretched out, it is about as long as a school bus! How do folds help the small intestine take up nutrients? 36 ENGAGE Materials Why is the small intestine full of folds? Make a Prediction One of the main jobs of the small intestine is to absorb water and nutrients. If you use paper to model the way the small intestine absorbs water, which type will absorb the most water? Make a prediction. • construction paper Test Your Prediction • ridged paper towels Pour the same amount of water into each graduated cylinder. Observe and record the water levels of the graduated cylinders. • plain paper towels • printer paper Fold the construction paper twice lengthwise. • water Dip the construction paper into a graduated cylinder until half is covered in water. • 4 graduated cylinders After 30 seconds remove the paper. Observe and record the water level in the graduated cylinder. • stopwatch Repeat steps 2 through 4 for the other paper types. Step Draw Conclusions Which type of paper absorbed the most water? Do these results support your prediction? Compare How does the structure of the paper that absorbed the most water compare to the small intestine? Infer What is the function of the small intestine? Explore More Step What do you think would happen if you repeated the experiment using a bath towel? Design and perform an experiment. Then share your results. 37 EXPLORE What does the digestive system do? ▶ Main Idea The digestive system breaks down food into nutrients that the body absorbs. ▶ Vocabulary nutrients, p. 38 digestive system, p. 38 enzyme, p. 38 metabolism, p. 42 All body cells need energy to stay alive and to do their jobs. This energy comes from food. Yet the food you eat every day comes in very big pieces. Cells are too small to use food as large as an apple or even one bite of an apple. Tiny food pieces that the body can use are called nutrients. The body needs many kinds of nutrients, and different foods are rich in different kinds. Eating a variety of foods will help you get all of the necessary nutrients. Food also contains materials that cannot be digested. Such material is called dietary fiber. Many grains, fruits, and vegetables contain fiber. It is removed from the body as solid waste. Even though it cannot be digested, dietary fiber is an important and healthful part of the diet. Water is also a part of many foods. Every body cell needs water, as does the blood. Digesting Food The digestive system is the set of organs that breaks down food into tiny pieces. The organs of the digestive system form a tube through the body. Food is broken down further and further as it moves along the tube. When food pieces are small enough, they are taken up into the blood. Food is digested in two ways. In mechanical digestion, food is torn or ground up into smaller parts. The teeth begin mechanical digestion. In chemical digestion, food is broken apart chemically. This happens in the mouth, the stomach, and the small intestine. The digestive system uses organs called glands to aid in chemical digestion. These glands make products called enzymes. An enzyme helps a chemical change take place. Enzymes for digestion come from glands in the mouth, the pancreas, and other organs. ▲ Both mechanical and chemical digestion begin as soon as you bite into a piece of food. 38 EXPLAIN Quick Check Critical Thinking Is all food taken up into the blood? Why or why not? Nutrients from Food Bread, rice, and pasta are rich in starches, which are types of carbohydrates. They are broken down into glucose, the molecule that cells use for energy. Fruits and vegetables have dietary fiber. They also contain many vitamins and minerals. Cells need these nutrients to carry out life processes. Meats and beans are rich in proteins, which are used for cell growth and repair. Milk products have calcium, an important mineral for healthy bones and teeth. Most foods are rich in some nutrients and poor in others. A healthful diet includes foods from all of the food groups. Oils are found in nuts, fish, and other foods. They contain lipids, a nutrient that includes fats and other substances. Body cells need a small amount of lipids. Read a Photo Why is it important to eat a variety of different foods? 39 EXPLAIN Digestion When you put food into your mouth, it begins a long journey through your digestive tract. First, your teeth grind and tear the food into smaller pieces. While this happens, glands in your mouth release saliva. Saliva is a mixture of water and enzymes. It softens food and begins the process of chemical digestion. Your tongue and muscles in the back of your throat help you swallow the mass of food, which is called a bolus (BOH•lus). This mass then moves down your esophagus (ih•SAH•fuh•gus), a long, thin tube that is lined with muscles. These muscles squeeze the bolus down to the next organ, the stomach. The stomach is a muscular pouch inside the belly. Here food is digested both mechanically and chemically. The stomach churns food back and forth, and it adds acids and enzymes that help break down proteins. The food is changed into a form called chyme (kighm). Chyme is like a thick soup. The Digestive System 1 1 Mouth The mouth is where digestion begins. 2 Esophagus The esophagus is a tube that connects your mouth to your stomach. 3 Liver The liver adds digestive juices to food. 2 3 4 Stomach The stomach is a hollow bag with muscular walls. 5 4 5 6 6 Gallbladder The gallbladder is a pear-shaped organ that stores digestive juices produced by the liver. 7 Small Intestine The small intestine connects the stomach and the large intestine. It absorbs nutrients. 8 Large Intestine The large intestine eliminates undigested waste. 7 8 40 EXPLAIN Pancreas The pancreas is an organ that produces several important enzymes. Depending on the kind of food eaten, the stomach may take several hours to release the chyme. The chyme enters the small intestine, a coiled tube where most chemical digestion takes place. Though it is called the small intestine, only the width of this tube is small. Its length is about 7 meters (23 feet)! Model the Small Intestine Materials: string, meterstick, scissors Near the beginning of the small intestine, enzymes from the pancreas and gall bladder are added into the chyme. Once these enzymes do their work, nutrients are ready to be absorbed into the blood. This happens in the many folds of the small intestine called villi (VIH•ligh). What to Do Food that is not absorbed passes into a wider, shorter tube called the large intestine. From here, water is absorbed into the blood. Most materials that remain will leave the body as waste. Draw Conclusions Measure Wrap string around the length of a meterstick to measure 7 meters. Cut the string and stretch it out. Carefully coil the string to a size that could fit inside your abdomen. Infer What does this model show about the way food moves through the small intestine? ◀ Villi of the small intestine soak up water and nutrients into the blood. They work much like tiny strands of cloth on a towel. Quick Check Critical Thinking Why does the small intestine need to be so long? 41 EXPLAIN Metabolism A huge number of chemical changes take place as the body breaks apart food and uses its energy. Together, these chemical changes make up the metabolism (muh•TA•buh•lih•zum) of the body. Metabolism includes the breaking down of food for its energy. It also includes the building up of new body tissues. Many factors affect metabolism. One factor is diet. As you know, food provides energy to the body. The energy in foods is measured in Calories. If the body takes in more Calories than it needs, it typically stores the extra Calories as fat. Exercise helps use up the energy from food. It also builds up muscle mass. Muscle cells, even when they are resting, use up many more Calories than fat cells. Exercise can increase a person’s overall metabolism by increasing his or her muscle mass. The body’s metabolism is always changing. It speeds up when the body exercises, and it slows during times of rest and sleep. It also slows gradually as the body ages. Sometimes the body’s metabolism does not work correctly. It releases too much or too little of a certain hormone. Metabolic disorders can often be treated with surgery or by taking oral or injected hormones. Quick Check Critical Thinking Why is it important to balance diet and exercise? ◀ A dietitian or nutritionist can help people choose the diet that’s right for them. 42 EXPLAIN Visual Summary Food provides the nutrients that body cells need. Many nutrients are used for energy. Others are used for growth and other life processes. The digestive system breaks down food into nutrients. Both mechanical and chemical digestion begin in the mouth. Think, Talk, and Write Main Idea What happens to food after it is eaten? Vocabulary Glands make ______ that aid in chemical digestion. Cause and Effect Why can professional athletes eat more food than most people eat, yet still not gain weight? Cause Effect Critical Thinking Compare actions of Glands make enzymes that aid chemical digestion. Villi are folds of the small intestine that absorb nutrients into the blood. Diet and exercise affect metabolism, which is the total set of chemical changes in the body. Writing Link the digestive system that are under a person’s control with those that happen automatically. Test Prep Breads and pasta are rich in A carbohydrates. B proteins. C fats. D vitamins. Test Prep Food passes through all of these organs EXCEPT the small intestine. esophagus. stomach. pancreas. A B C D Math Link Prepare for an Interview Calculate Ratios What if a company began advertising a product called “The Perfect Food”? Write questions to ask about this food. Describe why these questions and their answers are important. Research an animal’s body length and the length of its digestive tract. Then find the ratio of these lengths. Compare the ratios between plant-eaters, such as sheep, and meat-eaters, such as lions. 43 EVALUATE Lesson 5 Nervous System Nerves connect the brain with organs throughout the body. Just how fast can this system work? 44 ENGAGE Materials Can reaction time change? Form a Hypothesis Test your reaction time by trying to catch a falling object. What variables might affect your reaction time? Will you react faster to a sound or a sight? Write your hypothesis in the form: “If my eyes are shut, then my reaction will be….” Test Your Hypothesis • ruler Create a chart like the one shown to record your results. Hold a ruler vertically with your hand at the top of it. Have your partner put one hand near the bottom of the ruler without touching it. • paper • pencil Step Drop the ruler, and observe the spot where your partner caught the ruler. This is the control. The closer your partner’s hand is to the bottom number, the quicker the reaction. Repeat the test five times, making sure to record the data from each test. Repeat the activity, but have your partner close his or her eyes. Make a noise as you drop the ruler. This is the variable. Repeat the test five times and record your results. Draw Conclusions Interpret Data Compare the reaction times for the control and the variable. Do you notice any patterns? Was your hypothesis correct? Step Infer Why would reaction time vary according to whether a person’s eyes are open? Explore More How will reaction time change if your partner uses the other hand to catch the ruler? Repeat the experiment, and compare your result to those of the other teams in your class. 45 EXPLORE How do nerves work? ▶ Main Idea The nervous system controls thoughts and actions. ▶ Vocabulary If you put your ear next to a telephone line, would you hear people’s voices? Of course not. Telephone lines carry electrical signals. At the end of a line, a receiver translates those signals into sounds. In some ways the nervous system acts like telephone lines for the body. The nervous system uses nerves to carry messages quickly. The message of a single nerve is in the form of an electrical signal called an impulse. nervous system, p. 46 neuron, p. 46 Neurons spinal cord, p. 47 A nerve cell that carries impulses is called a neuron (NOOR•ahn). Impulses travel through a neuron in one direction only. brain stem, p. 48 cerebellum, p. 48 cerebrum, p. 48 reflex, p. 51 The tiny, fingerlike parts of a neuron are called dendrites. Dendrites carry impulses to the cell body, which is the central part of the neuron. The cell body, in turn, sends an impulse down the axon. The axon carries the impulse to the end of the nerve. Neuron Impulses ◀ Like a telephone line, a neuron carries messages in the form of an electrical signal. axon axon terminals cell body dendrites 46 EXPLAIN The ends of two neighboring neurons are very close but do not touch. The tiny space between them is called a synapse (SIH•naps). When an impulse reaches the end of an axon, a chemical is released across the synapse. This is how nerves communicate. The Nervous System brain A single impulse cannot be made stronger or weaker, nor can it be made faster or slower. However, a series of impulses can be sent down an axon at different rates. These rates determine the effect of the neuron. Parts of the Nervous System The nervous system has three main parts: the brain, the spinal cord, and peripheral (puh•RIH•frul) nerves. The spinal cord is a long column of nerves that extends from the base of the brain down the back. It acts to relay messages between the brain and the rest of the body. The brain and spinal cord together make up the central nervous system. All other nerves make up the peripheral nervous system. The word peripheral means “to the side.” Some peripheral nerves are quite long. The longest nerves span between the toes and the spinal cord. The brain is involved in almost every message that the nervous system carries. These messages may travel to the brain, from the brain, or entirely within it. Quick Check spinal cord peripheral nerves ▲ Most nerves that lead directly to or from the brain are part of the spinal cord. Peripheral nerves span across the rest of the body. Critical Thinking If nerves are like telephone lines, which body part acts like the telephone receiver? 47 EXPLAIN The Brain The adult brain is made of about 100 billion neurons. It is the body’s most complex organ—and one of the most important. The three largest parts of the brain are the brain stem, the cerebrum, and the cerebellum. Each of these parts, in turn, is divided into many smaller parts and regions. Although the parts do separate jobs, they all work together to control nearly every action of the body. The brain stem is the part closest to the spinal cord. It controls breathing, heart rate, the cycle of sleeping and waking, and other actions that keep the body alive. The cerebellum (ser•uh•BE•lum) helps control balance and posture. It receives messages from muscles and from the inner ear, which has a sense organ for balance. It uses all of this information to move muscles in the right way to keep the body from falling. The cerebrum (suh•REE•brum) is the largest part of the brain. It has the unique power to think, decide, and act. The Cerebrum: A Closer Look The outer surface of the cerebrum looks quite different from the layers beneath. The outer surface, called the cerebral cortex, is made of cell bodies of neurons that are packed closely together. This tissue is also known as gray matter. When you remember, think, or create, you are using your cerebral cortex. This is the part of your brain that interprets information from the senses, recognizes language, and is the source of many emotions. Rather than having a smooth surface, the cerebral cortex has many folds and grooves. This increases its surface area. Although other animals have larger skulls and heads, none have as much gray matter as humans do in proportion to the size of their brains. The Brain The cerebrum is responsible for thoughts, feelings, memories, and conscious motion of body parts. It has many lobes, each with its own function. The brain stem controls essential body functions and connects the brain to the spinal cord. 48 EXPLAIN The cerebellum keeps the body in balance. Beneath the gray matter is the white matter. It is made of axons of neurons. White matter carries messages between the gray matter and the rest of the body. In addition, certain regions help release hormones, as well as feel hunger, thirst, and other sensations. The brain’s cerebrum is divided into left and right halves called hemispheres (HE•muh•sfihrs). According to many scientists, the two hemispheres think in different ways. The left hemisphere is more logical, while the right hemisphere is more creative. Each hemisphere is divided into separate sections called lobes. Each lobe fills a specific role. One lobe is involved in vision. Another lobe is involved in speech, memory, and hearing. ▲ The cerebrum has gray matter on the outside and white matter on the inside. Gray matter allows intelligent thought. Quick Check Critical Thinking How can the brain do tasks without being aware of them? Although the cerebrum chooses how to move the skeleton, the cerebellum keeps it in balance. ▶ 49 EXPLAIN Peripheral Nerves Peripheral nerves travel throughout the body. Twelve pairs of them connect directly to the brain. All others connect to the spinal cord. Like other nerves, peripheral nerves carry messages in one direction only. Nerves that carry messages to the brain are called sensory nerves. These include nerves for sight, hearing, smell, touch, and taste. Other nerves carry information about the position of the body or the saltiness of the blood. Still others sense temperature, pain, and vibration. Anything that triggers a sensory nerve is called a stimulus (plural: stimuli). A touch to the skin, food on the tongue, and a painful injury are all examples of stimuli. Nerves that carry messages from the brain are called motor nerves. These nerves lead to muscles, including those that move bones, the heart, and other organs. Motor nerves and sensory nerves are connected by associative nerves, or interneurons. Sensory nerves send messages to the brain all the time. The brain processes these messages. In response it may send messages down motor nerves. Often it does so automatically, without awareness or thought. In many cases a message from the body may trigger an emotion, a desire, or a physical reaction. If the blood is too salty, the brain senses thirst. If a spot in the skin is dry or prickly, the brain senses an itch. A Reflex Action A tack pricks the finger (stimulus). Sensory neurons transmit an impulse to associative neurons. Associative neurons transmit an impulse to motor neurons. You pull your finger away from the tack (response). 50 EXPLAIN Motor neurons transmit an impulse to the muscles. Usually the brain is the only connection between sensory and motor nerves. The exception is a reflex. A reflex is a muscle movement that happens very quickly and automatically, without the involvement of the brain. Doctors can test some reflexes to make sure the nervous system is working correctly. Tapping a tendon in the knee with a small rubber hammer causes the knee-jerk reflex—the lower leg kicks outward. Tricks of Sight What to Do Observe Look at the diagram below. Compare the two yellow circles. Which appears larger? Record your observations. Sense Organs You see with your eyes, hear with your ears, and smell with your nose. These organs, however, do not work all by themselves. Nerves connect each organ directly to the brain. The brain interprets the information it receives. Each sense organ has nerves that respond to a special type of stimulus. The back of the eye, for example, has cells that change when light strikes them. The changes trigger signals in nerves that lead to the brain. A lobe in the cerebrum uses these signals to form a picture. Measure Using a ruler, find the width of each circle. Record your measurements. How do the sizes of the yellow circles compare? Observe Stare at any of the blue squares in the image below. Then look at the small white dots in the grid. How do they appear? Record your observations. The ear has tiny bones and other tissues that move in response to sound. A nerve connects the ear to the brain. The cerebrum interprets the information from this nerve as noise, music, words, or other kinds of sounds. The nose picks up tiny particles that hang in the air. The brain interprets these particles as an odor, or smell. Draw Conclusions Quick Check Critical Thinking How are reflexes Infer Do these pictures trick the eyes or the brain? Explain your answer. different from other actions of peripheral nerves? Why are reflexes useful? 51 EXPLAIN Nerve Injuries and Repair Some nerves are easy for the body to fix or replace. For example, when the skin is cut or scraped, nerve endings may be damaged or lost. They typically grow back and work normally again. In other cases, however, the body does not repair injured or dead nerves. Tissues in the brain and spinal cord are examples. A burst blood vessel in the brain can cause a stroke, an event that often kills brain tissues. Patients who suffer strokes may never regain lost brain function. Injuries to the brain and spinal cord can also be very serious. This is why helmets are so important when biking, skiing, or playing sports, such as football and hockey. When bones of the spine are injured or broken, they can press against or damage the spinal cord. This can affect the body’s ability to move. As a general rule, the higher up the spine that an injury strikes, the greater the region of the body it affects. Quick Check Critical Thinking How might a spinal ▼ Although some injuries to cord injury affect the ability to walk? the spinal cord may never heal, exercises and training can help patients recover. ▲ Helmets used in sports are designed to protect the head and brain from damage. 52 EXPLAIN Visual Summary Like telephone wires, the nervous system carries messages across the body. A neuron is a nerve cell that carries a message. The cerebrum, cerebellum, and brain stem are three parts of the brain. The cerebral cortex thinks, remembers, and imagines. Think, Talk, and Write Main Idea What is the role of the nervous system? Vocabulary Balance and posture are controlled by the ______. Sequence A boy feels thirsty and then drinks some water. Describe how his nervous system worked. Include at least four parts of the system. First Next Last Most peripheral nerves travel from the spinal cord to body parts. Nearly all carry messages to or from the brain. Sense organs respond to light, sounds, and other stimuli, and then send information through nerves to the brain. Writing Link Critical Thinking Why is it useful that the cerebral cortex stays unaware of many functions of the brain? Test Prep An action that involves peripheral nerves only is a muscle contraction. muscle movement. reflex. response to an emotion. A B C D Test Prep Where is the gray matter in the brain? A on the cerebrum B on the cerebellum C in the right hemisphere D in the left hemisphere Math Link Book Report Estimate Volume Many science-fiction stories involve creatures with very powerful brains or senses. Find such a story from the library. In your report, compare the human brain to the brain in the story. Propose a plan to estimate the volume of your brain. Use a ruler or measuring tape and the formula for the volume of a sphere or a rectangular prism. Discuss your plan with a partner, and then try it. 53 EVALUATE Wide Nerves for Giant Squids Invertebrates are animals without backbones. Most invertebrates’ bodies do not grow very long. But that is not the case for squids. The giant squid is the world’s largest invertebrate. The longest recorded giant squid was 18 meters (59 feet) long. In 2007, New Zealand fishermen caught a colossal squid, which is not as long as a giant squid, but much wider and often much heavier—this one weighed 450 kilograms (990 pounds)! ▼ A giant squid uses its tentacles to capture fish and other prey. Wide, fast-acting nerves allow this. 54 EXTEND Animal Bodies Nerves wrapped in myelin help vertebrates grow very tall or very long.▼ How can a squid grow so big? One reason is that it has extremely thick axons, or nerve fibers. Thick axons carry impulses much faster than narrow nerves do. By using nerves that communicate quickly, the squid’s brain can control distant body parts. Many vertebrates also grow tall or long, yet their nerves are not unusually thick. A white substance called myelin (MY•uh•luhn) speeds up the rate of nerve impulses. Myelin is made of a layer of proteins between two layers of lipids. It forms a sheath, or coating, around axons. A myelin sheath keeps the impulse contained in a small area around the axon. This means that the chemicals that transmit the impulse from one nerve cell to another do not have far to move. Write About It Summarize Using the Internet or other resources, research additional information on how different kinds of animals have different nerves. Make a poster displaying your findings. Include a graph or chart that shows animals’ sizes. 55 EXTEND Lesson 6 Excretory System The kidneys have millions of tiny bumps like these. How does the structure of the kidneys help them remove wastes from the body? 56 ENGAGE Materials How do your kidneys filter out waste? Purpose Your kidneys filter waste out of your blood using a special kind of membrane. The membrane has very small openings that allow some molecules to pass through but not others. Waste particles are kept inside the kidneys and useful substances are sent back into the body. Make a model of a kidney’s membrane. • • cornstarch • measuring spoon Procedure • water Experiment Mix 1 teaspoon of cornstarch and • eye dropper 200 mL of hot water in a beaker. • 2 beakers Mix 150 mL of water and 5 mL of iodine in the other beaker. • spoon Pour 50 mL of the cornstarch water into the bag and close it with a twist tie. • iodine Gently place the bag in the iodine solution without letting the twisted top get wet. • twist ties • cellophane bag Step Observe Check the beaker every 3 minutes for 15 minutes. Write down your observations. Draw Conclusions What happened to the cornstarch in the bag? Why do you think this happened? How is the bag similar to the membrane of the kidneys? Explore More Step Would salt filter through the bag? What about pepper? Why might one substance move through the bag, but not another? How does this compare to the kidneys’ function? Explain. 57 EXPLORE How does the excretory system work? ▶ Main Idea Several systems eliminate body wastes, including solids, liquids, and gases. ▶ Vocabulary excretory system, p. 58 urinary system, p. 58 liver, p. 58 kidney, p. 60 nephron, p. 60 Cells throughout the body produce wastes, or materials the body cannot use. Removing these wastes is the job of the excretory (EK•skruh•tor•ee) system. Unlike other body systems, the excretory system includes parts that work separately from each other. These parts include the following: • the urinary system, which filters the blood to remove liquid wastes • the end of the digestive system, which removes solid wastes from food • the respiratory system, which removes carbon dioxide • the integumentary system, or skin, which removes water and salts through sweat Wastes in the Blood ▼ Sweat glands in the skin are part of the excretory system. Many kinds of wastes collect in the blood. Carbon dioxide is a waste gas that most cells produce. Other wastes come from old cells or cell parts. Even medicines can form wastes as they are broken down in the body. Poisons may enter the body, too. A poison in the blood is called a toxin. Toxins may come from the bite or sting of snakes, bees, spiders, and other animals. Toxins can also come from harmful bacteria. The liver treats many of these wastes and toxins. The liver is a large, reddish-brown organ in the middle of the body, just below the diaphragm. The liver does not remove wastes directly. It changes them into products that can be removed more easily later. One toxic waste is a compound called ammonia. The liver changes it into urea (yoo•REE•uh), which is the waste material in urine. Quick Check Critical Thinking Could the body survive without an excretory system? Explain. 58 EXPLAIN The Urinary System Liver The liver treats wastes and toxins in the blood. Kidneys These two organs filter wastes from the blood and produce urine. Bladder The bladder stores urine until it can be released. Ureters Tubes carry urine from the kidneys to the bladder. Urethra This tube carries urine away from the bladder and out of the body. 59 EXPLAIN All of your blood circulates through your kidneys about 60 times a day. The kidneys are a pair of bean-shaped organs located near your back just above your waist. They control the amount of chemicals in the blood. Urine collects near the center of the kidney. From there it flows through tubes called ureters (YOO•ruh•turz) to the bladder. The bladder is a muscular bag that holds urine. The brain tells the bladder when to empty. Urine leaves the body through a tube called the urethra (yoo•REE•thruh). Inside each kidney are millions of tiny filtering tubes called nephrons (NE•frahnz). Nephrons get blood from capillaries and separate waste from useful materials. They send the waste into a larger collecting duct, or tube. The waste is processed into urine. Kidneys also control the amount of water in the blood. If the blood is too watery, the kidneys put the extra water into the urine, making it lighter-colored. If the blood is low on water, the kidneys will make darker-colored urine with less water. The Kidneys The Kidney artery capillary vein nephron renal artery collecting duct renal vein ureter Read a Diagram Where do substances from the nephrons go? 60 EXPLAIN Health and the Urinary System Kidneys often work well throughout a person’s lifetime. The body can even live with only one working kidney. However, diseases of the kidneys and the urinary system are common. Some can be serious. Bacteria and other invaders sometimes travel from the outer surface of the skin into the lower part of the urinary system. This is called a urinary tract infection. These infections are especially common in women. Women have shorter urethras than men, so the bacteria do not need to travel as far to reach the bladder. Many urinary tract infections last only a short time and cause only mild pain or discomfort. However, an infection that reaches the kidneys can cause serious damage. Kidney stones are hard pieces of salts and minerals that build up in the urinary system. Large stones can block a ureter. Passing them can be painful. Kidney stones are more common among older adults than other age groups. Drinking lots of water and eating well can help keep kidneys healthy. Wearing the proper safety equipment when playing sports is also important. Although a layer of fat protects the kidneys, a hard blow to the back or belly can injure them. Quick Check Critical Thinking Why do most infections of the urinary system leave the kidneys unharmed? Comparison of a Full and an Empty Bladder ▲ With help from a special dye, X-ray pictures can show the urinary system. These pictures show an empty bladder and a full bladder. 61 EXPLAIN Dialysis and Transplants Filtering with Water Materials: paper towel, clear plastic cup, water, food dye, sugar cube What to Do Color the sugar cube with food dye. The colored cube models wastes in the blood. Cover the cup with a paper towel. Place the colored cube on it. Drip water over the cube. As the water collects in the cup, observe the changes to its color. Draw Conclusions Infer How does water help the kidneys filter wastes? A dialysis machine acts like an artificial kidney. Dialysis has saved many lives. ▼ Kidneys sometimes stop working completely. This is called kidney failure. One way to treat this problem is with dialysis. Dialysis (digh•AH•luh•sus) is a treatment that artificially filters the blood to remove wastes. In one common dialysis method, blood is channeled through tubes that carry it into a machine. The machine is filled with a sugary liquid that draws wastes out of the tubes. Dialysis is only a temporary solution. Treatments usually must be repeated three times every week and each treatment takes several hours. Because dialysis cannot remove all waste products, dialysis patients have restricted diets. A permanent way to treat a faulty kidney is to transplant a new one. To transplant is to move an organ from one person’s body to another. Doctors have had success with many kidney transplants but not in all cases. Even with the proper medicine, the body sometimes rejects an organ that is not its own. Transplants usually work best when the replacement organ comes from a close relative. A person can live a normal life with only one working kidney. For this reason, people can choose to donate a kidney to a patient with kidney failure. Quick Check Critical Thinking Why is dialysis only a temporary solution to kidney failure? 62 EXPLAIN Visual Summary The excretory system removes wastes from the body. Unlike other body systems, it includes parts that work separately from each other. The kidneys filter wastes from the blood to make urine. They are part of the urinary system. Think, Talk, and Write Main Idea What is the job of the excretory system? Vocabulary The kidney is made of millions of tiny filtering structures called ______. Sequence Explain how the liver, kidneys, and bladder work in sequence to remove wastes from the blood. First Next Last A nephron is a structure in the kidney that makes urine. The kidneys use millions of nephrons. Diseases and injuries can damage the kidneys. Dialysis filters blood artificially, or a new kidney can be transplanted. Writing Link Test Practice The skin is part of the excretory system because it makes A carbon dioxide. B sweat. C vitamins. D urea. Test Practice Nephrons filter blood from ______ in the kidneys. A arteries B veins C capillaries D alveoli Art Link Research Report Kidney Poster Research a disease that affects the liver or the kidneys. Find out its cause, effects, and treatment. Write a report to show what you learned. Make a poster that illustrates the kidneys and how they work. Include labels and captions. 63 EVALUATE Lesson 7 Endocrine and Reproductive Systems During a close race, an athlete may feel both nervous and excited. How might emotions affect how an athlete performs? 64 ENGAGE How do emotions affect the body? Step Make a Prediction An emotion is a feeling, such as happy, sad, excited, angry, or frightened. How do emotions affect the way the body works? Record your prediction. Emotion Test Your Prediction Excited Make a chart like the one shown. Record data Think of times when you felt the emotions listed in the chart. Complete the chart by answering the question in each column. What other emotions have you felt recently? Add a row to the chart for each emotion, and then repeat step 2. What was I doing when I felt this emotion? I was running a race. Why did I feel this way? What was happening in my body? I came from behind to win! My heart and breathing rates were fast, and my leg muscles were working hard. Bored Frightened Draw Conclusions Analyze data Review your answers. When do the heart, the lungs, and the muscles work the hardest? When do they work more slowly? Infer How do body systems Relaxed Angry respond to emotions? Cite evidence to support your answer. Explore More How could your emotions help you perform better in a race or a game? Make a prediction, and design a way to test it. 65 EXPLORE What does the endocrine system do? ▶ Main Idea Hormones of the endocrine system change how body parts function. The changes may be temporary or permanent. ▶ Vocabulary endocrine system, p. 66 hormone, p. 66 adrenaline, p. 68 insulin, p. 68 reproductive system, p. 70 The organs of most body systems are joined closely together. The glands of the endocrine (EN•duh•krin) system, however, are scattered throughout the body. The endocrine system works to send messages through the blood to organs of other systems. Its glands, or groups of special cells, make chemicals called hormones to carry these messages. Many body systems depend on hormones to work properly. The nervous system also sends messages. But hormones work differently. They travel more slowly, and their effects usually last much longer. Blood carries hormones from the endocrine glands throughout the body. Yet each type of hormone changes only certain kinds of cells, and it changes them only in certain ways. In some cases a hormone triggers cells to begin working or reproducing. In other cases it signals cells to stop working or reproducing. The effects of a hormone may last a few hours, a few years, or a lifetime. Hormones help muscles to use food, bones to grow longer, and mothers to carry and give birth to babies. If there is a problem with an endocrine gland, it may make too much or too little of a hormone. Both situations can be harmful to the body. Sometimes a doctor can restrict the production of a hormone if a person’s body is producing too much of it. He or she can also give extra hormones if a person’s body is not making enough. ◀ Human growth hormone makes bones grow longer. In some people the result is greater than in others. Quick Check Critical Thinking How is the endocrine system similar to other body systems? How is it different? 66 EXPLAIN The Endocrine System Hypothalamus This gland lies deep in the brain. Its hormones control the pituitary gland. Adrenal glands These glands are near the kidneys. They make adrenaline to give cells extra energy. Pituitary gland Some of its many hormones affect growth, blood pressure, childbirth, and the function of other glands. Pancreas It makes two hormones that control the amount of glucose in the blood. Parathyroid glands They produce a hormone that moves calcium from bones into the blood. Ovaries These organs in women make hormones that cause the body to develop female body traits. Thyroid gland Its hormones help cells use energy. Testes These organs in men make hormones that cause the body to develop male body traits. Read a Diagram Which glands control the body’s energy use? 67 EXPLAIN Many Hormones The endocrine system includes many glands that make a huge number of hormones. The glands respond to changes in the blood or to messages from the nervous system. Adrenal glands Have you ever noticed that your heart beats faster when you feel frightened? The adrenal (uh•DREE•nool) glands are the cause. They make a hormone called adrenaline (a•DREN•uh•lin), which is also called epinephrine (eh•puh•NEH•frun). Adrenaline makes the heart and lungs work faster. It also alters the amount of blood flow to some body parts and it releases glucose into the blood. Together these actions help the body use muscles to move quickly. Adrenaline is often nicknamed the “fight or flight” hormone. Adrenaline affects the body in the same way whether a threat is real or not. ▲ Insulin shots help people with diabetes lead normal lives. Pancreas After a meal the pancreas (PAN•kree•us) releases insulin (IN•suh•lun). Insulin is a hormone that allows glucose to move from the blood into cells. Along with other hormones, insulin controls the amount of glucose in the blood. Too much glucose in the blood can damage the eyes, the kidneys, and other organs. If the level of glucose in the blood gets too high, the pancreas releases more insulin. Too little glucose starves cells of the energy they need. If the blood’s glucose level gets too low, the pancreas releases less insulin. When a person has a disease called diabetes (digh•uh•BEE•teez), the body either does not make or cannot use insulin properly. Type 1 diabetes is caused by a lack of insulin. This kind of diabetes often occurs in childhood. In type 2 diabetes, the body’s tissues do not respond to insulin. This kind of diabetes usually happens to adults. People with type 1 diabetes often need to take insulin injections. A person with severe diabetes can also have a pancreas transplant. 68 EXPLAIN Thyroid A gland called the thyroid (THIGH•royd) is located in the neck. It releases several hormones that help the body use energy. To make these hormones, the thyroid needs a chemical element called iodine (IGH•oh•dine). Iodine is a natural part of many foods. It is also added to salt. A faulty thyroid may produce either too many or too few of its hormones. Too many of them may cause weight loss and nervous, jittery feelings. Too few may cause weight gain and exhaustion. Pituitary Gland Some endocrine glands make only one or two kinds of hormones. The pituitary (puh•TEW•uh•tayr•ee) gland, however, makes at least nine different hormones. For this reason, it is often called the master gland. Model Insulin Materials: pitcher of water; food dyes colored yellow, blue, and green What to Do Make a Model Work in a small group. Add a few drops of green dye to the water. This color models healthy blood. Use Variables Use blue dye to model glucose and yellow dye to model insulin. Stir in a few drops first of blue dye, then of yellow dye, then of blue dye again. Observe the results. As a group, decide how to restore the green color that models healthy blood. Carry out your plan. Draw Conclusions Infer How does this activity model the role of insulin in the blood? Pituitary hormones affect bones, kidneys, and organs of the reproductive system. Some affect other glands, including the thyroid and the adrenal glands. Quick Check Critical Thinking Why does the body need many kinds of hormones, rather than just one or two? This mass in the neck is called a goiter. Lack of iodine caused the thyroid to grow larger. ▶ pituitary gland ◀ This gland is about the size of a pea, yet it makes many different hormones. 69 EXPLAIN The Reproductive System To reproduce is to make more individuals of the same species. The job of the reproductive system is to allow people to have children. All organs of the reproductive system are present throughout life, even at birth. They do not begin to fully develop until the end of childhood. The male and female reproductive organs are quite different. Testes (TES•teez) are organs in men that make sperm cells. Ovaries (OH•vuh•reez) are organs in women that make egg cells. A sperm and an egg unite to form the first cell of a new child. Ovaries and testes also act as endocrine glands. The hormones they produce help develop the reproductive system and the body traits of an adult woman or man. A woman who is carrying a developing baby is described as pregnant. Like nearly all female mammals, a pregnant woman carries the developing baby inside her body. The baby develops inside the woman’s uterus (YEW•tuh•rus), an organ just above the pelvis. The uterus expands to many times its normal size as the baby grows and develops. ▲ The first cell of this baby came from the union of two cells: a sperm cell from the father and an egg cell from the mother. A typical pregnancy lasts for about 9 months. Then the baby is born. Quick Check Critical Thinking How does the reproductive system differ from most body systems? 70 EXPLAIN Visual Summary Glands of the endocrine system make hormones. Hormones travel through the blood to change how organs work. Think, Talk, and Write Main Idea What is the role of the endocrine system in the body? Vocabulary A hormone called ______ helps move glucose from the blood into body cells. Sequence Choose a hormone and describe its path through the body. Endocrine glands are scattered across the body. Some are as small as a pea. Others are much larger. First Next Last Adrenaline is the “fight or flight” hormone. Insulin helps move glucose into the blood. The reproductive system allows adults to have children. Its organs are different in men and women. Writing Link Critical Thinking Does the size of an endocrine gland relate to its importance? Include an example. Test Practice Which of these glands is located in the neck? A thyroid B adrenal gland C pancreas D pituitary gland Test Practice The job of the uterus is to A make egg cells. B make sperm cells. C carry a developing baby. D make growth hormone. Social Studies Link Research Report Goiters Research a disease of the endocrine system, such as diabetes, thyroid disease, or Addison’s disease. Find out about the causes, effects, and treatment for each disease. An enlarged thyroid is called a goiter. Goiters are common in environments where iodine is rare. Find out where these places are and how goiters can be prevented there. 71 EVALUATE Lesson 8 The Human Life Cycle As the human body ages, it changes in predictable ways. About how old are the people in this photograph? How can you tell? 72 ENGAGE Materials How do boys and girls grow differently? Make a Prediction Children and early teens grow taller every year. Do you think boys and girls of your age grow differently from one another? Make a prediction. Test Your Prediction • meterstick or measuring tape Measure Work in pairs. Remove your shoes, keep • calculator your feet flat on the floor, and have your partner measure your height. Switch roles to measure the height of your partner. Record Data The teacher will draw a chart on Step the board with two columns—one for boys and one for girls. Record your height in the appropriate column. Use Numbers Calculate the average height of boys in the class. First add their heights together, then divide by the number of boys. Repeat step 3 for the girls in the class. Draw Conclusions Interpret Data Who is taller in your class, the boys or the girls? How much taller are they? Predict How do you think the height difference will change during the next few years? Explain your prediction. Explore More Why do some people grow taller than others? Do diet, ethnicity, and genetics make a difference? Research human growth and present your results to the class. 73 EXPLORE How does the body change over time? ▶ Main Idea As the body ages, it grows and develops through the stages of the human life cycle. ▶ Vocabulary life cycle, p. 74 adolescence, p. 77 puberty, p. 77 A newborn baby girl is small enough for her parents to hold in their arms. Over time she will grow into an older child, then a teenager. When she is an adult, she may choose to have a baby of her own. Changes like these happen in a predictable pattern that all human beings follow. This pattern is called the human life cycle. A cycle is a series of events that happen over and over. Although an individual human’s life has a beginning and an end, reproduction ensures that new lives continue the cycle. The human life cycle includes many stages that occur at characteristic times of life. Before Birth fetus at 20–24 weeks of development A human body is made of a huge number of cells. Yet those cells all come from a single cell. This special type of cell, called a zygote, forms when a sperm cell and an egg cell unite. The cell divides into two cells, which divide into four cells, then eight cells, and so on. Inside the body of the mother, cells begin dividing in a tube that leads from an ovary. Then the cells attach to the wall of the uterus. During this time, the group of cells is called an embryo (EM•bree•oh). The uterus and embryo are connected by a structure called the placenta (plu•SEN•tuh). The placenta brings food and oxygen from the mother to the developing baby. As the embryo grows, early versions of body tissues and organs are formed. After 8 weeks, the embryo starts to resemble a baby, even though it is only about 1 inch long! At this stage, the embryo has become a fetus (FEE•tus). ▲ A fetus gradually develops organs and body systems needed for life outside the uterus. 74 EXPLAIN Over many months, body parts develop that will eventually allow the fetus to survive outside the mother’s body. The fetus grows a skeleton and muscles, as well as about 100,000 brain cells every minute. Infancy A baby is born after about 9 months of development in the uterus. At birth, a newborn uses his or her lungs and a few other organs for the first time. Young babies are called infants. Infants need their parents or other adults to care for them. They cannot eat solid food. Instead they rely on their mothers’ milk or formula for nutrition. When their needs are not met, they will cry loudly for help. Infants grow and develop rapidly. The brain continues to develop, as do bones and muscles. By 1 year old, most babies will have tripled in weight, grown some teeth, and be able to eat some solid food. Many babies will have learned a few words and are able to recognize other people. Quick Check Critical Thinking A fetus has developing ▲ The umbilical cord connects the fetus lungs. Why are they not used until birth? to the placenta. When it falls off, it leaves a scar called the navel, or belly button. ◀ More growth and development takes place in the first year than in any other. 75 EXPLAIN Childhood Growing Up What to Do Classify Which stage of life are you in now? Explain how you know. Make a timeline of important events in your life. Begin with the day you were born. Include other events that you were told took place or that you remember. Draw Conclusions Predict Add to your timeline by predicting future events. Childhood lasts from the end of infancy to about age 12 or 13. With every year, children learn new knowledge and skills. They also take on new responsibilities in their lives. However, children still rely on adults to provide food, shelter, and protection. Very young children are called toddlers. To toddle is to walk with short, wobbly steps. Toddlers are still developing the muscle control and balance that they need to walk and run properly. Children learn a great deal from interacting with the world around them. Young children begin to understand the language that they hear every day. They also learn to use words to express their thoughts and needs rather than by crying as infants do. Young children start to learn cause and effect by experimenting. They drop toys or other objects to watch them fall. Simple games of peekaboo teach them that just because they cannot see an object does not mean it is gone forever. Older children are more agile than young children. They lose their baby teeth and start to grow permanent teeth. Older children are able to do many things that adults do. They develop their muscles by playing games and sports. They develop their minds by reading books and studying in school. They also develop their emotions by building relationships with friends and family members. ◀ By age 5, the muscular and nervous systems are well developed. Children can walk, run, climb, dance, and learn new ways to move their bodies. 76 EXPLAIN Adolescence After childhood, children enter adolescence (a•dul•ES•unts). During adolescence, the body grows taller and begins to develop the traits of an adult. The time that adolescence begins is called puberty (PYEW•bur•tee). Many girls enter puberty at around age 12 or 13, although it can start at an earlier or later age. Boys typically enter puberty about 2 years after girls do. Adolescence is triggered by changes in hormones from the endocrine system. In girls, the ovaries release a hormone called estrogen (ES•truh•jun). Estrogen causes the breasts and female reproductive organs to develop. It also helps begin a process of the female reproductive system called the menstrual (MEN•stroo•ul) cycle. In boys, the testes release a hormone called testosterone (tes•TAHS•tuh•rohn). This hormone deepens the voice and causes male reproductive organs to develop. Other body changes involve the skin. During adolescence, the skin makes more sweat and may have acne, or pimples. Hair also begins growing under the arms, in the pubic area, and on boys’ faces. Adolescents often sleep longer than they did as children. Their bodies are growing and changing very quickly, which can be tiring. Quick Check Critical Thinking How are children different from adolescents? How are they alike? Entering Puberty age 7 age 12 Girls and boys enter puberty at different ages. Adolescence brings many changes to the body and the mind. age 17 Read a Photo How do the boys’ and girls’ heights compare in each of these photos? 77 EXPLAIN ▲ Humans of all ages can learn from each other and from their own experiences. Adulthood When adolescence ends, the body has stopped growing taller and all body systems have developed fully. The body is described as mature, or fully grown. Adulthood is the mature stage of the human life cycle. Adults may choose to have children. By having children, they are continuing the human life cycle. Their children will grow and develop, just as they did. A woman may continue to give birth to children as long as her menstrual cycle continues. The menstrual cycle stops at a time called menopause (MEN•uh•pawz). Women often reach menopause in their 40s or early 50s. The body of an adult may appear to change very little from year to year. Yet cells always change as they age. Gradually cells lose their ability to divide and reproduce. While important body systems continue working, many slow down. By age 60 or 70, the body shows many signs of a long life. Wrinkles may line the skin, and hair may turn gray. Years of use, illnesses, and diseases may weaken many body parts. Many people live into their 80s or 90s. Death is the end of life. It occurs when body systems stop working permanently. Quick Check Critical Thinking How does the adult body change as it ages? 78 EXPLAIN Visual Summary Humans grow and develop in stages of the human life cycle. Birth occurs after nine months of development inside the mother’s body. Childhood is a stage of rapid growth of both the body and the mind. Older children can do many things that adults can do. During adolescence, the body grows taller and takes on the traits of the adult. Reproductive organs develop in both boys and girls. In adults the body gradually slows down as it ages. By age 60 or 70, the body shows many signs of a long life. Writing Link Think, Talk, and Write Main Idea List the stages of the human life cycle. Vocabulary Adolescence begins at a time called ______. Cause and Effect Why might older adults suffer from more illnesses and diseases than younger people? Cause Effect Critical Thinking Does everyone pass through the stages of the human life cycle in the same way? Explain. Test Prep A developing baby is attached to the uterus by the placenta. fetus. embryo. adolescent. A B C D Test Prep Adolescence is caused by changes in the muscular system. skeletal system. endocrine system. immune system. A B C D Art Link Write a Magazine Article Make a Collage How do children, adolescents, and adults compare? Write a magazine article that answers this question. Include your classmates’ ideas as well as facts about stages of the life cycle. Look in magazines, newspapers, or online for photographs of people in different stages of life. Make a collage to show these stages, and include labels and captions. 79 EVALUATE At the Hospital At a large hospital, hundreds of patients are treated every day. Some patients are seriously ill. Others only need some special attention. Many people give birth in hospitals. Specially trained doctors and nurses help with the birth and can check the health of the baby and the mother. People with serious injuries or illnesses go to the emergency room. Heart attacks, broken bones, and severe asthma are just a few of the conditions that emergency room doctors treat on a daily basis. ▼ In the hospital, newborn babies stay either in a nursery or in rooms with their mothers. 80 EXTEND the nursery ▲ Some hospitals treat only children. Others care for children and adults in separate sections. Science, Technology, and Society ◀ Anyone who needs help right emergency room away is brought to the emergency room. Doctors save many lives here. Hospitals have physical therapy facilities. Special exercises help patients recover from injuries, diseases, or other events that weaken bones and muscles. ▼ Sometimes patients need help getting strong again after an illness or an injury. Hospitals often have physical therapy facilities where trainers work with people to build up their strength, balance, and flexibility. Some hospitals only treat certain types of patients. Hospitals that only treat children are called pediatric hospitals. There are also hospitals that specialize in treating certain organs, such as the heart. Others treat certain diseases, such as cancer. Write About It Research What else can you learn about hospitals? Do any have departments that specialize in all of the different body systems? Research using an encyclopedia or other reference materials, and write a short summary. 81 EXTEND Lesson 9 The Body’s Defenses When germs invade the body, white blood cells work to stop them. How can germs spread from one person to the next? 82 ENGAGE How quickly can germs spread? Materials Make a Prediction Read steps 1 through 4. How many students will get the germ? Make a prediction. Test Your Prediction Your teacher will give each student a pair of index cards. Two students will receive cards marked with an X. An X represents a germ. All other cards are marked with a circle. • 2 index cards per student Make a Model Walk around the room. When • pencils or markers your teacher says “stop,” find the student nearest you. Exchange one card with this student. Observe your new card. If it has an X anywhere on it, add an X to both of your cards. Otherwise add a circle to both. Step Repeat steps 2 and 3. Complete five rounds. After the fifth round, count the number of students who have the germ. Draw Conclusions Use Numbers How accurate was your prediction? If more rounds were added, would all students receive the germ eventually? Explain. Infer What did the activity model about the way germs spread? Explore More What if four students acted to stop the spread of germs? If they stop trading cards once they receive an X card, predict how the results will change. 83 EXPLORE What causes disease? ▶ Main Idea Diseases can harm the body. They are fought by the body’s defenses, along with medicine and medical technology. ▶ Vocabulary A disease is an illness or a condition in which the body does not work properly. Any organ or body system can have a disease. Many diseases can attack at any stage of the life cycle. A symptom is a sign that the body has a disease. A runny nose, a fever, or an upset stomach are common symptoms of many diseases. Infectious Diseases infectious disease, p. 84 immune system, p. 86 white blood cell, p. 86 antibody, p. 87 antibiotic, p. 88 deer tick Harmful germs are called pathogens (PA•thuh•jenz). They can be bacteria, fungi, viruses, or parasites, such as protists. Pathogens that invade a person’s body can cause infectious diseases (in•FEK•shus di•ZEE•zuz). The word infect means “to invade.” Infectious diseases that can spread from person to person are also called communicable (kuh•MYEW•ni•kuh•bool) diseases. Animals can spread pathogens both to one another and to humans. A disease called rabies, for example, is transmitted by the bite of an infected animal. People can get malaria and West Nile virus from the bites of infected mosquitoes. Deer ticks can carry the bacteria that cause Lyme disease. Pathogens may spread through air, water, and food, as well as from person to person. Many germs can be spread by a sneeze or a cough. Covering your mouth when sneezing or coughing helps to stop germs from spreading. So does washing your hands after touching a doorknob or other object that many people touch. Infectious Diseases Pathogens Diseases Bacteria strep throat, tetanus, tuberculosis, plague, Lyme disease, whooping cough, typhoid fever Fungi ringworm, athlete’s foot, toenail fungus, valley fever, Darling’s disease (histoplasmosis) Viruses flu, chicken pox, cold, polio, rabies, measles, smallpox, AIDS, yellow fever, warts Protists malaria, African sleeping sickness, dysentery, giardiasis, black fever, Chagas’ disease 84 EXPLAIN ▲ The use of tobacco products can cause many diseases. Warnings on their labels are required by law. Many Diseases, Many Causes Not all diseases are infectious. Some diseases pass from parents to children through genes. Cystic fibrosis is one example. When a person has cystic fibrosis, his or her body makes a substance called mucus that is unusually thick and sticky. The mucus causes problems in the lungs and the digestive system. Other diseases strike when body parts wear down after years of use. For example, the ends of bones are lined with cartilage. As this cartilage wears away, joints begin to hurt. This disease is called arthritis. Many older people have problems with arthritis in their knees, fingers, and other joints. Some diseases happen suddenly. If blood stops flowing to a person’s brain, it results in a stroke. A stroke may be caused by a clot that clogs a blood vessel, blocking the flow of blood. It can also happen when a blood vessel breaks and leaks into the brain. A stroke keeps oxygen and nutrients from getting to brain cells, causing them to die within a few minutes. ▲ Cholesterol is a material in fatty foods. It can build up inside arteries and cause heart disease. Poor health habits can lead to many diseases. A diet with too many fatty foods, for example, can make arteries stiff and narrow. This makes the heart work harder. The heart may fail suddenly in an event known as a heart attack. Smoking can cause diseases of the heart and lungs. Both smoking and chewing tobacco increase the risk for developing cancer. Cancer is a dangerous disease that has many different causes. It can strike any organ. Most types of cancer are not infectious. Cancer starts when a group of cells divide incorrectly and rapidly. The cells may form a mass called a tumor. Cancer cells and tumors can spread across the body, sometimes traveling to distant body parts. Scientists continue to study the different causes of cancer in hopes of preventing it. Many types of cancer can now be treated, and a few can be cured. Quick Check Critical Thinking Why is it that some diseases are infectious while others are not? 85 EXPLAIN Fighting Disease Fortunately, the body has ways to prevent diseases. It can also fight off many diseases when they attack. The Immune System To fight pathogens, the body uses the immune system. The immune system is made up of different components. If one component fails, the next one takes over. The body’s first line of defense is the integumentary system. The skin acts like a wall to stop germs from invading. Tears wash germs out of the eyes. Nose hair, mucus, and earwax trap germs. Saliva and stomach acid can kill germs, too. Despite these defenses, germs often find paths into the body. A wound in the skin is one such path. If germs get into the body, another portion of the immune system takes action. Have you ever gotten very hot when you were sick? A fever is a rise in body temperature that also helps to fight invaders. It slows the growth of some pathogens and helps infection-fighting cells do their jobs. Some cells in the blood have the job of fighting off germs that attack the body. They form a group of cells called white blood cells. Some types of white blood cells are best at killing bacteria. Others are best at killing viruses. white blood cell ◀ Certain kinds of white blood cells begin working to defend the body as soon as germs invade. 86 EXPLAIN Two special types of white blood cells are called T cells and B cells. These cells travel throughout the body, fighting invaders wherever they meet them. Vaccines T and B cells work together to make antibodies. Antibodies are proteins that can attach to pathogens. The pathogens are then broken apart, either by other proteins or by other white blood cells. Specific antibodies are made in response to specific pathogens. The immune system keeps a few copies of every antibody it has ever made. There are also certain T cells that “remember” specific pathogens. This means that if the same pathogen attacks the body again, B cells would quickly make copies of the proper antibodies. This explains why people get some diseases, such as chicken pox, only once in their lives. Doctors use vaccines to prepare the immune system for fighting serious diseases. A vaccine is made of a pathogen that is dead or weakened so that it cannot damage the body. When white blood cells meet the pathogen in the vaccine, they make antibodies against it. This makes them ready to fight the real pathogen if it should get into the body. Vaccines have helped stop deadly diseases that were once quite common, such as smallpox and polio. Scientists are working to develop new vaccines, too. Quick Check Critical Thinking Why can some pathogens infect the body only once? 5 White blood cells break up the pathogen. Immune System at Work 2 T cell is copied. 1 T cell identifies pathogen. 3 Some T cells attack infected cells. Other T cells tell B cells to make antibodies. 4 Antibodies attach to pathogens. 87 EXPLAIN Treating Disease The body can fight many diseases on its own. It can rid itself of a cold or the flu, for example, in a few days. However, the body needs help to fight more serious diseases. A medicine or drug is a substance that changes how the body works. Vaccines are medicines that prevent disease. Cough syrup and aspirin treat the symptoms of disease, such as cough and fever. Other medicines help the body fight the cause of a disease. ▲ All medicines must be used properly. You should only take medicines under adult supervision. Some medicines must be taken only under a doctor’s supervision. These are called prescription medicines. The doctor prescribes, or orders the use of, a particular medicine. He or she tells the patient very carefully how the medicine is to be used and the results to expect. Some medicines may be used without an order from a doctor. These are called over-the-counter medicines. Any adult may purchase this kind of medicine. An antibiotic is a medicine that kills bacteria. The first antibiotic was penicillin (pen•uh•SIL•un). It was discovered in 1928, and over time it has saved many lives. Many other antibiotics are now widely used. All antibiotics are prescription medicines. Antibiotics do not kill viruses, however. Illnesses caused by viruses, such as the common cold, cannot be treated with antibiotics. ◀ Pharmacists are medicine experts. They fill prescriptions and help people use them. 88 EXPLAIN Other Treatments Some diseases cannot be treated with medicine alone. In many cases the body may need surgery (SUR•juh•ree). A surgery is also called a surgical operation. It is usually performed by a doctor called a surgeon. In surgery, a surgeon uses special tools to open the body and treat the disease. A surgery may involve removing, replacing, or repairing damaged tissues or organs. Surgery must be performed under sterile conditions. To be sterile (STAYR•ul) is to be free of germs of all kinds. Sterile conditions keeps germs from infecting the patient’s body during surgery. Read the Label What to Do Read the label shown below. Discuss the information on it. Analyze Data What is the name of the medicine? How often should the patient take it, and for what reasons? Draw Conclusions Infer Why do you think this medicine must be prescribed by a doctor? Town Drug Patient: Smith, Mary Treating cancer may require a combination of medicine, surgery, and radiation. In radiation, a beam of energy is used to kill cancer cells. Quick Check Critical Thinking What diseases could not be treated with antibiotics? 123 Elm Street Empire, NV 89405 (775)555-0123 Prescriber: Berman, J.S. TAKE 1 TABLET UP TO 2 TIMES A DAY AS NEEDED FOR DIZZINESS. Meclizine, 12.5 mg tablets Date filled: 08-24-2008 Quantity: 30 tablets 2 refills allowed ◀ By keeping conditions sterile, a surgeon can open the body without causing an infection. 89 EXPLAIN ▲ Penicillin, the first antibiotic, was widely used for many years. Yet it gradually lost its effectiveness against many diseases. Antibiotic Resistance Unfortunately antibiotics become less effective over years of use. Penicillin, for example, once killed all kinds of bacteria. Today it affects very few of them. Doctors no longer use penicillin in its original form. What caused this change? When penicillin was discovered, only a few bacteria could survive its effects. Those few bacteria are said to be resistant to penicillin. The resistant bacteria survived and made copies of themselves. Over time they became more common. Resistant bacteria will grow in number in response to almost any antibiotic. For this reason doctors use the newest antibiotics only when older ones no longer work. They want the new medicines to last as long as possible. 90 EXPLAIN ▲ Some strains of bacteria are sensitive to antibiotics, and others are resistant. The diameters of the clear rings show how effective the samples of antibiotics were. When a doctor prescribes antibiotics, they should be taken exactly as ordered. This includes taking all of the medicine, not skipping doses, and not sharing the medicine with others. Even though you might start feeling better after one dose of antibiotics, you should take all of the medicine prescribed. The first dose or two of antibiotics may kill some bacteria, but not all of them. Resistant bacteria might require many more doses of antibiotics. Some antibiotics must be taken for several days, weeks, or even months, depending on the type of illness. This helps to ensure that all of the bacteria are killed. Quick Check Critical Thinking Why shouldn’t people take antibiotics without a doctor’s order, even if they feel sick? Visual Summary Diseases have many causes. Diseases that are caused by pathogens are infectious diseases. Think, Talk, and Write Main Idea What fights disease in the human body? Vocabulary A medicine that fights bacteria in the body is called a(n) ______. Classify How can diseases be White blood cells of the immune system make antibodies to fight invading pathogens. Treatments for disease include medicine, medical technology, and surgery. Antibiotics help the body fight bacteria, but they become less effective over years of use. Writing Link classified into two groups? Give three examples for each group. Critical Thinking With every vaccine there is a very small risk of infection. Why are vaccines used anyway? Test Prep Which is a rise in the body’s temperature in response to germs? A antibody B fever C vaccine D radiation Test Prep To be sterile is to be free of all living things. white blood cells. germs. air and water. A B C D Social Studies Link Interview an Expert History Report Interview a doctor or a nurse about antibiotics. Ask which antibiotics they use most often and which they use only when necessary. Write a report to discuss what you learned. Research the discovery of the smallpox vaccine by Edward Jenner. Be sure to identify the kind of pathogen that caused this disease. Present your findings to the class. 91 EVALUATE Use this glossary to learn how to pronounce and understand the meanings of Science Words used in this book. Page numbers tell you where to find words in the book. Pronunciation Key The following symbols are used throughout the Macmillan McGraw-Hill Science Glossaries. a ā ä âr ô at ape far care law e ē i ī îr end me it ice pierce o ō ôr oi ou hot old fork oil out u ū . u up use rule ū pull ûr turn hw ng th th zh white song thin this measure ә about taken pencil lemon circus ' = primary accent; shows which syllable takes the main stress, such as kil in kilogram (kil' ә gram'). ' = secondary accent; shows which syllables take lighter stresses, such as gram in kilogram. aorta (ā ôr'tә ) The wide artery that leads out of the heart. (p. 29) Blood travels through the aorta on its way to the head, arms, or legs. adolescence (ad'ә les'ә ns) The stage of growth that begins at puberty and occurs between childhood and adulthood. (p. 77) The body develops adult traits during adolescence. artery (är'tә rē) A blood vessel that carries blood away from the heart. (p. 28) An artery throbs as blood rushes through it. adrenaline (ә dre' nә ’lә n) A hormone that is released into the bloodstream in response to mental or physical stress. (p. 68) Both excitement and fear can cause a release of adrenaline. axon (ak'sôn) The long part of a neuron that carries impulses away from the cell body. (p. 46) An impulse travels as a wave down an axon. alveoli (äl’vē ō’lī) Tiny air sacs in the lungs that exchange gases with the blood. (p. 27) Air travels through tubes that lead to alveoli. antibiotic (an'tē bī ot’ik) A medicine that kills or slows the growth of bacteria. (p. 88) Penicillin was the first antibiotic. antibody (an'ti bod’ē) A molecule in the immune system that recognizes or attaches to a specific pathogen. (p. 87) White blood cells can make an antibody against a virus. anvil (an'vә l) One of three tiny bones in the middle ear. (p. 16) The anvil carries sounds from the hammer to the stirrup. 92 GLOSSARY B cell — cerebrum calorie (kal'ә rē) A measurement of the amount of energy in food. (p. 42) Foods in the grain group provide many calories per serving. B cell (bē sel) Special white blood cell that works with T cells to make antibodies and to fight pathogens. (p. 87) Each type of B cell recognizes a unique pathogen. bladder (blad'ә r) Part of the excretory system, a sac that holds urine. (p. 60) The bladder releases urine into the urethra. blood pressure (blud presh'ә r) The force of moving blood against the inner walls of blood vessels. (p. 31) Blood pressure that is too high can lead to heart disease. capillary (kap'ә ler'ē) A short, thin, blood vessel that delivers blood to body cells. (p. 27) A capillary connects a small artery to a small vein. carbohydrate (kär’bō hī'drāt) A nutrient that provides energy for the body (p. 39) Starches and sugars are types of carbohydrates. cartilage (kär'tә lij) A firm, elastic body tissue that is a part of the skeletal system. (p. 16) Parts of the ear, nose, and trachea are made of cartilage. cell body (sel bod’ē) The central part of a neuron. (p. 46) Messages travel from the cell body down the axon. cell (sel) The basic unit of living matter, or tissue. (p. 4) All living things are made up of cells. central nervous system (sen'trә l nûr'vә s sis'tә m) Th e brain and spinal cord. (p. 47) All nerves in the body connect to the central nervous system. cerebellum (sә r'ә bel' um) The part of the brain that coordinates movement and controls balance and posture. (p. 48) The cerebellum helps muscles work together to move the body effectively. brain stem (brān stem) The part of the brain that connects to the spinal cord. (p. 48) The brain stem automatically controls many body functions. bronchi (brong' kī) A pair of tubes that branch off the trachea and lead to the lungs. (p. 27) Bronchi connect the lungs to the upper part of the respiratory system. calcium (kal'sē ә m) A mineral needed especially by bones and teeth. (p. 39) Milk is a good source of calcium. cerebral cortex (sә rē' brel surface of the cerebrum, also known as gray matter. (p. 48) Thinking and imagination take place in the cerebral cortex. kor'teks) The outer, folded cerebrum (sә rē' brum) The part of the brain where thought, memory, and muscle control take place. (p. 48) A well-developed cerebrum allows humans to be intelligent. 93 GLOSSARY chemical digestion — epidermis chemical digestion (kem'i kә l dī jes'chә n) The chemical breakdown of food. (p. 38) Saliva in the mouth begins chemical digestion. diaphragm (dī'ә fram) A long, flat muscle that helps move air in and out of the lungs. (p. 27) The size of the chest changes as the diaphragm moves up and down. chyme (chīm) A thick fluid into which food is changed by the stomach. (p. 40) The stomach releases chyme into the small intestine. dietary fiber (dī'it âr’ē fī'bә r) Material from plants that cannot be digested. (p. 38) Whole-grain cereals, fruits, and vegetables are good sources of dietary fiber. circulatory system (sûr'kyә lә tôr’ē sis'tә m) The body system responsible for moving blood through the body. (p. 28) The heart and blood vessels make up the circulatory system. communicable disease (kә mū'ni kә bә l di zēz') A disease that can spread from one person to another. (p. 84) Covering your mouth when you sneeze can stop communicable diseases from spreading. compact bone (kom' pakt bōn) The tissue that forms the hard outer layer of bones. (p. 19) Compact bone gives the arm and leg bones the strength they need. dendrite (den'drīt) Tiny, branch-like part of a neuron that receives signals from other cells. (p. 46) The end of a neuron is lined with dendrites. dermis (dûr'mīs) The layer of skin below the epidermis. (p. 22) Nerve endings, blood vessels, and sweat glands are part of the dermis. diabetes (dīә bē'tēz) A disease caused by an inability to make or use insulin properly. (p. 68) People with diabetes often take shots of insulin. dialysis (dī al' ә ’sis) A medical procedure that artificially removes wastes from the blood. (p. 62) Dialysis takes over the job of faulty kidneys. 94 GLOSSARY digestive system (dī jes'tiv sis'tә m) The body system that breaks down food into nutrients. (p. 38) The organs of the digestive system form a long tube through the body. electrocardiogram (i lek’tro kär'dē o gram) A graphic recording of heart muscle activity. (p. 32) Doctors can detect heart disease by studying an electrocardiogram. embryo (em'brē ō) The stage of human development that begins shortly after an egg is fertilized. (p. 74) A spine begins to form in the embryo. endocrine system (en dô'krin sis'tә m) The body system that makes and sends chemical messengers called hormones. (p. 66) The thyroid, thymus, and pituitary glands are organs of the endocrine system. enzyme (en'zim) A type of protein in living things that helps chemical changes take place. (p. 38) An enzyme in the stomach helps break down proteins. epidermis (ep’i dûr'mīs) The outer layer of the skin. (p. 22) The epidermis sheds thousands of skin cells every day. esophagus — iodine esophagus (i sof'ә gә s) The tube that carries food from the mouth to the stomach. (p. 40) Chewing food thoroughly allows it to pass through the esophagus. heartbeat (härt'bēt’) One complete pulsation of the heart. (p. 32) Your doctor can hear your heartbeats with a stethoscope. estrogen (es trō' gin) A female sex hormone that is released by the ovaries. (p. 77) Estrogen signals female traits to develop. hemisphere (hem'ә sfîr’) The left or right half of the brain. (p. 49) Different kinds of thinking take place in the left and right hemispheres of the brain. excretory system (ek skrә tôr'ē sis'tә m) The body system that removes wastes. (p. 58) Kidneys are important organs of the excretory system. hemoglobin (hē mō' glōb in) The material in red blood cells that binds to oxygen and gives blood its red color. (p. 30) The blood’s hemoglobin contains iron. fetus (fē'tә s) The stage of development of a baby before birth. (p. 74) A fetus develops all of the body systems that a baby needs. hormone (hôr'mōn) A chemical messenger that travels through the blood. (pp. 6, 66) A hormone from the pituitary gland controls how the body grows. immovable joint (i müv'ә bә l joint) A joint where no motion occurs. (p. 18) The skull is an example of an immovable joint. immune system (i mūn' sis'tә m) The body system that recognizes and fights specific pathogens. (p. 87) The immune system uses special white blood cells to make antibodies. impulse (im'puls) The message of a single nerve, in the form of an electrical signal. (p. 46) Impulses travel through neurons to carry messages to, from, and within the brain. fever (fē'vә r) An abnormally high body temperature. (p. 86) Fever helps the body fight infections. glucose (glü' kōs) A type of sugar that cells use for food. (p. 4) Blood carries glucose to every body cell. infectious disease (in fek'shә s di zēz') A disease caused by a pathogen, or harmful germ, that invades the body. (p. 84) Colds, the flu, and strep throat are examples of infectious diseases. insulin (in'sә lin) A hormone from the pancreas that controls the levels of sugar in the blood. (p. 68) Insulin levels may be very high after a heavy meal. integumentary system (in teg' yә men tә rē sis'tә m) The body system that covers the body and includes skin, hair, and nails. (p. 8) The integumentary system acts as a first line of defense against germs. hammer (ham'ә r) One of three tiny bones in the middle ear. (p. 16) The hammer carries sound waves from the eardrum to the anvil. iodine (ī'ә dīn’) A chemical element that is used by the thyroid to make hormones. (p. 69) Lack of iodine can lead to an enlarged thyroid gland. 95 GLOSSARY joint — nervous system joint (joint) The place where two bones meet. (p. 18) The elbow joint allows the arm to bend like a hinge. keratin (ker'ә tin) A tough, fibrous protein that makes up hair and nails. (p. 22) Specialized skin cells secrete keratin. kidney (kid'nē) A bean-shaped organ that filters wastes from the blood and produces urine. (p. 60) Tiny blood vessels and filtering tubes make up a kidney. mature (mә chür') Fully grown. (p. 78) The human body is mature when it has stopped growing taller. marrow (mâr'ō) Soft, fatty tissue inside many bones. (p. 19) Blood cells are made by bone marrow. mechanical digestion (mi kan'i kә l dī jes'chәn) The physical process of breaking down food into smaller parts. (p. 38) The teeth begin mechanical digestion of food. menopause (men'ә pôz) The time in a woman’s life when the menstrual cycle stops. (p. 78) Many women reach menopause between the ages of 45 and 55. menstrual cycle (men'strü ә l sī’kә l) A monthly cycle of the female reproductive system. (p. 77) Many hormones control the events of the menstrual cycle. metabolism (mә tab'ә liz’ә m) The set of chemical processes that occur in living things. (p. 42) Metabolism gradually slows as the adult body ages. large intestine (lärj in tes'tin) The wide tube at the end of the digestive tract. (p. 40) Undigested parts of food pass through the large intestine and out the body. life cycle (līf sī’kә l) The stages of growth, development, and reproduction of a species. (p. 74) Childhood, adolescence, and adulthood are three stages of the human life cycle. ligament (lig'ә mә nt) Tough, rope-like tissue that connects bones or cartilage at a joint. (p. 18) A ligament may break if a bone is pushed too hard. lipid (lip'id) A nutrient that is part of fat. (p. 39) The body needs small amounts of many kinds of lipids. liver (liv' ә r) A large, reddish-brown organ that treats wastes and toxins so that the body can remove them. (p. 58) Urea is a waste product made in the liver. lymph (limf ) A pale fluid around body tissues that contains white blood cells. (p. 30) A system of ducts carries lymph around the body. 96 GLOSSARY mineral (min'ә r ә l) A nutrient that helps the body’s chemical processes. (p. 39) Iron, iodine, and calcium are important minerals for the human body. motor nerve (mō'tә r nûrv) A nerve that carries messages from the brain to muscles or other body parts. (p. 50) A muscle receives instructions through a motor nerve. muscular system (mus'kyә lә r sis'tә m) The body system that moves bones and other body parts. (p. 20) Exercise will build up the strength and flexibility the muscular system. nephron (nef'rôn) The filtering unit of the kidney. (p. 60) The kidneys use millions of nephrons to filter the blood. nervous system (nûr'vә s sis'tә m) The body system responsible for thinking, remembering, and carrying messages to and from body parts. (p. 46) Nerves of the nervous system spread to nearly all body parts. neuron — reflex neuron (nûr’rôn) A cell that carries messages in the nervous system. (p. 46) A neuron may carry messages to, from, or within the brain. nutrient (nū’trē ә nt) A material in food that the body uses for energy or to stay healthy. (p. 38) Carbohydrates, vitamins, and minerals are types of nutrients. organ system (ôr’gә n sis'tә m) A set of organs that work closely together to meet one or more of the body’s needs. (p. 6) The lungs are part of an organ system for breathing. ovaries (ō'vә rēz) The organs in women that produce egg cells and female sex hormones. (p. 70) Ovaries mature and begin working during adolescence. over-the-counter medicine (ō'vәr thә koun'tәr med'ә sin) A medicine that any adult may purchase. (p. 88) Aspirin and cough syrups are popular over-the-counter medicines. placenta (plә ’sen'tә ) The organ that connects a developing baby to the uterus of the mother. (p. 74) A developing baby receives food and oxygen through the placenta. platelets (plāt'lets) Cell fragments in the blood that help blood clot. (p. 30) Platelets clump together at the site of a wound. pregnant (preg'nә nt) To have a baby developing inside the body. (p. 70) A woman is pregnant for about 9 months before the baby is born. prescription medicine (pri skrip'shә n med'ә sin) A medicine that can be obtained only by a doctor’s order. (p. 88) A doctor orders a prescription medicine to treat strep throat. Town Drug Patient: Smith, Mary 123 Elm Street Empire, NV 89405 (775)555-0123 Prescriber: Berman, J.S. TAKE 1 TABLET UP TO 2 TIMES A DAY AS NEEDED FOR DIZZINESS. Meclizine, 12.5 mg tablets Date filled: 08-24-2008 Quantity: 30 tablets 2 refills allowed pathogen (path'ō jin) Any disease-causing germ, including many bacteria and viruses. (p. 84) Three lines of defense protect the body against pathogens. protein (prō'tēn) A nutrient that helps body tissues grow and repair themselves. (p. 39) Beans, meat, and fish are good sources of proteins. peripheral nervous system (pә ri' fu’rùl nûr'vә s sis'tә m) The nerves that lead into or out of the brain or spinal cord. (p. 47) Nerves in the arms and legs are part of the peripheral nervous system. puberty (pū'bûr tē) The time at which adolescence begins. (p. 77) Sexual traits begin to develop during puberty. pituitary gland (pi tü'ә gland) A gland that regulates much of the endocrine system. (p. 69) Hormones from the pituitary gland affect many other glands. radiation (rā’ dē ā' shә n) A medical treatment that uses a beam of energy to kill cancer cells. (p. 89) Radiation can be used in combination with medicine and surgery to treat cancer. târ’ē reflex (rē flecks') A body movement that occurs without the involvement of the brain. (p. 51) Jerking a hand away from a hot stove is an example of a reflex. 97 GLOSSARY reproductive system — synapse reproductive system (rē’prә duk'tiv sis'tә m) The body system that allows adults to have children. (p. 70) Organs of the reproductive system differ between men and women. resistant (ri zis’tә nt) Describes bacteria that cannot be killed by antibiotics. (p. 90) Bacteria that are resistant to penicillin have become more common. respiratory system (res'pә r ә tôr’ē sis’tem) The body system responsible for breathing. (p. 26) The lungs are the main organs of the respiratory system. spinal cord (spī'nә l kôrd) A thick bundle of nerves held inside the spine. (p. 47) Nerves from the arms and legs connect through the spinal cord to the brain. sterile (ste'rùl) Free of disease-causing germs. (p. 89) Surgery must be performed under sterile conditions. sternum (stûrn' um) A flat bone that connects to the ribs and protects the chest. (p. 16) The sternum helps give shape to the chest. stimulus (stim'yә lus) Any event that triggers a sensory nerve. (p. 50) A stimulus can be interpreted as a sound, sight, touch, taste, or smell. stirrup (stûr'ә p) One of three tiny bones in the middle ear. (p. 16) The stirrup works with the hammer and anvil to carry sounds. surgeon (sur’jen) A doctor who opens the body and performs a surgery. (p. 89) A surgeon may be asked to remove a diseased organ. surgery (sûr'jә rē) An operation or procedure to remove, repair, or replace a body part. (p. 89) Cancer patients often need surgery to remove a tumor. saliva (sәl i'vә ) A liquid that wets food in the mouth and helps break it apart both physically and chemically. (p. 40) The smell of food can lead to the mouth releasing saliva. surgical operation (sûr'ji kә l op’ә rāsh'ә n) A medical procedure performed by a surgeon that is used to treat disease or repair the body. (p. 89) Cancer and other diseases can be treated by performing a surgical operation. sensory nerve (sen'sôr’ē nûrv) A nerve that carries messages into or toward the brain. (p. 50) Information from a sense organ travels through a sensory nerve to the brain. serum (sîr'ә m) The liquid part of blood. (p. 30) Glucose is carried in the serum of the blood. skeletal system (skel'i tùl sis'tә m) A framework of bones that shape and support the body. (p. 16) Muscles move the skeletal system at joints. small intestine (smôl in tes'tin) The long, narrow tube where most chemical digestion takes place. (p. 40) Nutrients are absorbed into the blood by the lining of the small intestine. 98 GLOSSARY symptom (simp'tә m) A sign that a disease is present. (p. 84) Pain and fever are symptoms of many diseases. synapse (sin' aps) The tiny space between a neuron and another cell, across which chemical messages travel. (p. 47) Chemicals need a fraction of a second to travel across a synapse. T cell — villi T cell (tē sel) A type of white blood cell that works with B cells to make antibodies and to fight pathogens. (p. 87) T cells help stop pathogens from invading more than once. urea (yûr'ē ә ) A waste material in urine. (p. 58) Urea is made by the liver and removed by the kidneys. urinary system (yûr'i nâr’ē sis'tә m) The part of the excretory system that makes and removes urine. (p. 58) Kidneys, the bladder, and several tubes are part of the urinary system. urine (yûr'in) The liquid produced by the kidneys to remove body wastes. (p. 8) The bladder releases urine from the body. uterus (ū tә r'is) The organ in women in which a baby grows and develops. (p. 70) The placenta is attached to the wall of the uterus. tendon (ten'dә n) A tough, cord-like tissue that attaches a muscle to a bone. (p. 20) A tendon pulls on a bone when a muscle contracts. testes (tes' tēz) The organs in men that produce sperm cells and male sex hormones. (p. 70) Testes develop fully during adolescence. testosterone (tes'tôs’ tә r ōn) Male sex hormones that are released by the testes. (p. 77) Testosterone causes the voice to deepen. thyroid (thī' roid) An endocrine gland in the neck that affects how the body uses energy. (p. 69) A faulty thyroid could cause feelings of tiredness or anxiety. toxin (tok'sin) A poisonous substance in the blood. (p. 58) The liver changes many toxins into harmless wastes. trachea (trā' kē ә ) The tube that carries air between the throat and the lungs. (p. 26) A flap of tissue stops food from entering the trachea. transplant (trans plant') To transfer an organ from one person’s body to another. (p. 62) A patient may have a kidney transplant to replace faulty kidneys. vaccine (vak sēn') A medicine that prepares the immune system to fight a pathogen. (p. 87) The vaccine against small pox has saved many lives. vein (vān) A blood vessel that carries blood to the heart. (p. 28) Blood from most body parts drains into two veins that lead to the heart. vertebrae (vûr'tә brā) Disk-shaped bones that make up the spine. (p. 16) Lower vertebrae are thicker and wider than upper vertebrae. villi (vil'ī) Tiny bumps that line the wall of the small intestine and that absorb nutrients into the blood. (p. 40) Villi act like a towel to soak up nutrients. 99 GLOSSARY vitamin — white blood cell vitamin (vī'tә min) A nutrient that helps body cells work properly. (p. 39) Fruits and vegetables are good sources of many vitamins. white blood cell (hwīt blud sel) One of many types of cells that help fight infections. (p. 86) The body responds to many diseases by making more white blood cells. 100 GLOSSARY Credits The McGraw-Hill Companies, Inc. would like to acknowledge the artists and agencies who participated in illustrating this program: Argosy and Four Lakes Colorgraphics. Photography Credits: Front Cover (b)Dan McCoy/Rainbow/Getty Images, (l)Macmillan/The McGraw-Hill Companies; 2-3 F. Schussler/PhotoLink/Getty Images; 3 (t)Macmillan/The McGraw-Hill Companies, (b)Richard Hutchings/Digital Light Source; 4 MM Productions/CORBIS; 5 Image Source; 6 (tc)Joe Polillio/Macmillan/The McGraw-Hill Companies, (tr bl bc bl)Macmillan/The McGraw-Hill Companies; 7 (tl br)Joe Polillio/Macmillan/The McGrawHill Companies, (tc tr)Macmillan/The McGraw-Hill Companies, (bl bc)Richard Hutchings/Digital Light Source; 8 Tracy Frankel/Getty Images; 9 (b)Kelly Redinger/Design Pics/CORBIS, (t)Macmillan/The McGraw-Hill Companies; 10 (t to b)Stockbyte/PunchStock, (2)Al Telser/Macmillan/The McGraw-Hill Companies, (3)Jim Wehtje/Getty Images, (4)Stockdisc/PunchStock; 11 (t to b)MM Productions/CORBIS, (2)Macmillan/The McGraw-Hill Companies, (3)Richard Hutchings/Digital Light Source, (4)Tracy Frankel/Getty Images; 12 (t)Blend Images, (b)Randy Faris/CORBIS; 13 Dynamic Graphics/ Jupiterimages; 14-15 Louis Fox/Getty Images; 15 Richard Hutchings/Digital Light Source; 16 Dynamics Graphics Group/Creatas/Alamy; 17 Richard Hutchings/Digital Light Source; 18 Brand X/CORBIS; 19 Images-USA/Alamy; 20 (c)Alistair Berg/Getty Images, (bl)Innerspace Imaging/Photo Researchers, Inc., (bc)Carolina Biological Supply Company/Phototake, (br)Innerspace Imaging/Photo Researchers, Inc.; 21 (t)Richard Hutchings/Digital Light Source, (b)Steven Errico/Getty Images; 22 (l)Macmillan/The McGraw-Hill Companies, (r)Bananastock; 23 (tl)Richard Hutchings/Digital Light Source, (cl)Innerspace Imaging/Photo Researchers, Inc., (bl)Macmillan/The McGraw-Hill Companies; 24-25 Anatomical Travelogue/Photo Researchers; 25 Macmillan/The McGraw-Hill Companies; 26 Duomo/CORBIS; 27 Macmillan/The McGraw-Hill Companies; 28 Macmillan/The McGraw-Hill Companies; 30 (l)Al Telser/Macmillan/The McGraw-Hill Companies, (r)Comstock/CORBIS; 31 (tl)Don Farrall/Getty Images, (cr)Richard Hutchings/Digital Light Source; 32 (t)Hans Neleman/Getty Images, (b)Peter Widmann/Alamy; 33 (t)Duomo/CORBIS, (b)Comstock/CORBIS; 34 Bettmann/CORBIS; 35 (t)Bruce Ayres/ Getty Images, (b)Hank Morgan/Photo Researchers, Inc.; 36-37 Susumu Nishinaga/Photo Researchers; 37 Macmillan/The McGraw-Hill Companies; 38 PNC/Digital Vision/Getty Images; 39 (tl)Ingram Publishing/Alamy, (tr)Ingram Publishing/SuperStock, (cl)Ernie Friedlander/Cole Group/Getty Images, (c)Alan Richardson/Stockfood Creative/Getty Images, (cr)D. Hurst/Alamy, (bl)Image Source/Getty Images; 40 Joe Polillio/Macmillan/The McGraw-Hill Companies; 41 (l)Dr. Richard Kessel and Dr.Gene Shih/Visuals Unlimited, (r)Joe Polillio/Macmillan/The McGraw-Hill Companies; 42 liquidlibrary/ PictureQuest; 43 (tl)PNC/Digital Vision/Getty Images, (cl)Joe Polillio/Macmillan/The McGraw-Hill Companies, (bl)liquidlibrary/PictureQuest; 44-45 Dan McCoy/Rainbow/Getty Images; 45 (t)Richard Hutchings/Digital Light Source, (b)Joe Polillio/Macmillan/The McGraw-Hill Companies; 46 Royalty-Free/ CORBIS; 47 Macmillan/Mcgraw-hill Companies; 49 (t)BSIP/Photo Researchers, (b)Dex Image; 50 Macmillan/The McGraw-Hill Companies; 52 (l)Comstock Images/Jupiterimages Corporation, (r)Lawrence M. Sawyer/Getty Images; 53 (tl)Royalty-Free/CORBIS, (cl)Macmillan/The McGraw-Hill Companies; 54-55 AP Photo/Tsunemi Kubodera of the National Science Museum of Japan, HO; 55 Anup Shah/Photodisc/Getty Images; 56-57 Dr. Dennis Kunkel/ Visuals Unlimited; 57 Macmillan/The McGraw-Hill Companies; 58 David R. Frazier Photolibrary, Inc./Alamy; 59 Macmillan/The McGraw-Hill Companies; 60 Dr. Dennis Kunkel/Visuals Unlimited; 61 Science Photo Library/Photo Researchers; 62 AJPhoto/Photo Researchers; 63 (tl)David R. Frazier Photolibrary, Inc./Alamy, (tcl bcl)Dr. Dennis Kunkel/Visuals Unlimited, (bl)AJPhoto/Photo Researchers; 64-65 Jim Cummins/Getty Images; 66 Gary Caskey/Reuters/CORBIS; 67 (tl)Ken Cavanaugh/Macmillan/The McGraw-Hill Companies, (tr)Richard Hutchings/Macmillan/The McGraw-Hill Companies; 68 (t)Stockdisc/PunchStock, (b)Ron Fehling/Masterfile; 69 (t)Macmillan/The McGraw-Hill Companies, (b)Royalty-Free/CORBIS, (br)Scott Camazine/ Photo Researchers, Inc.; 70 BananaStock/PictureQuest; 71 (tl)Gary Caskey/Reuters/CORBIS, (cl)Ken Cavanaugh/Macmillan/The McGraw-Hill Companies, (cl)Ron Fehling/Masterfile, (bl)BananaStock/PictureQuest; 72-73 Paul Barton/CORBIS; 73 Richard Hutchings/Digital Light Source; 74 Brand X Pictures/ PunchStock; 75 (t)Don Tremain/Getty Images, (b)Cydney Conger/CORBIS; 76 IT Stock/PunchStock; 77 (l)Jupiterimages/BananaStock/Alamy Images, (c)George Doyle and Ciaran Griffin/Getty Images, (r)Amana Images, Inc./Alamy Images; 78 Royalty-Free/Masterfile; 79 (tl)Brand X Pictures/PunchStock, (cl)Amana Images, Inc./Alamy Images, (cl)IT Stock/PunchStock, (bl)Royalty-Free/Masterfile; 80 (t)Royalty-Free/CORBIS, (b)Photodisc Collection/Getty Images; 81 Dynamic Graphics/Jupiterimages; 82-83 Dennis Kunkel/Phototake; 83 (t)Macmillan/The McGraw-Hill Companies, (b)Richard Hutchings/Digital Light Source; 84 Scott Camazine/Photo Researchers, Inc.; 85 (l)James Leynse/CORBIS, (r)Kevin A. Somerville/PHOTOTAKE, Inc./Alamy Images; 86 Dr. Fred Hossler/Getty Images; 88 (t)Image Source/CORBIS, (b)Royalty-free/CORBIS; 89 Royalty-free/CORBIS; 90 (l)Bettmann/CORBIS, (r)Hank Morgan/Science Photo Library; 91 (tl)Scott Camazine/Photo Researchers, Inc., (cl)Royalty-free/CORBIS, (bl)Bettmann/CORBIS; 93 (cl)Don Farrall/Getty Images, (bl)D. Hurst/Alamy; 94 (tr)Alan Richardson/Stockfood Creative/Getty Images, (l b)Macmillan/The McGraw-Hill Companies; 95 (r)Comstock/ CORBIS, (l)Brand X Pictures/PunchStock; 96 Dr. Dennis Kunkel/Visuals Unlimited; 97 (t)PNC/Digital Vision/Getty Images,(b)Royalty-Free/CORBIS; 98 (l)Digital Light Source/Richard Hutchings, (r)Royalty-free/CORBIS; 99 (t)Macmillan/The McGraw-Hill Companies, (b)Dr. Richard Kessel and Dr. Gene Shih/Visuals Unlimited; 100 (t)Ingram Publishing/SuperStock, (b)Dr. Fred Hossler/Getty Images. 101 CREDITS