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Digestive System
The digestive system is made up of the digestive tract—a series of hollow organs joined in a long, twisting tube
from the mouth to the anus—and other organs that help the body break down and absorb food (see figure).
Organs that make up the digestive tract are the mouth, esophagus, stomach, small intestine, large intestine—also
called the colon—rectum, and anus. Inside these hollow organs is a lining called the mucosa. In the mouth,
stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digest food. The
digestive tract also contains a layer of smooth muscle that helps break down food and move it along the tract.
Two “solid” digestive organs, the liver and the pancreas, produce digestive juices that reach the intestine
through small tubes called ducts. The gallbladder stores the liver's digestive juices until they are needed in the
intestine. Parts of the nervous and circulatory systems also play major roles in the digestive system.
Why is digestion important?
When you eat foods—such as bread, meat, and vegetables—they are not in a form that the body can use as
nourishment. Food and drink must be changed into smaller molecules of nutrients before they can be absorbed
into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are
broken down into their smallest parts so the body can use them to build and nourish cells and to provide energy.
How is food digested?
Digestion involves mixing food with digestive juices, moving it through the digestive tract, and breaking down
large molecules of food into smaller molecules. Digestion begins in the mouth, when you chew and swallow,
and is completed in the small intestine.
Movement of Food Through the System
The large, hollow organs of the digestive tract contain a layer of muscle that enables their walls to move. The
movement of organ walls can propel food and liquid through the system and also can mix the contents within
each organ. Food moves from one organ to the next through muscle action called peristalsis. Peristalsis looks
like an ocean wave traveling through the muscle. The muscle of the organ contracts to create a narrowing and
then propels the narrowed portion slowly down the length of the organ. These waves of narrowing push the
food and fluid in front of them through each hollow organ.
The first major muscle movement occurs when food or liquid is swallowed. Although you are able to start
swallowing by choice, once the swallow begins, it becomes involuntary and proceeds under the control of the
nerves.
Swallowed food is pushed into the esophagus, which connects the throat above with the stomach below. At the
junction of the esophagus and stomach, there is a ringlike muscle, called the lower esophageal sphincter, closing
the passage between the two organs. As food approaches the closed sphincter, the sphincter relaxes and allows
the food to pass through to the stomach.
The stomach has three mechanical tasks. First, it stores the swallowed food and liquid. To do this, the muscle of
the upper part of the stomach relaxes to accept large volumes of swallowed material. The second job is to mix
up the food, liquid, and digestive juice produced by the stomach. The lower part of the stomach mixes these
materials by its muscle action. The third task of the stomach is to empty its contents slowly into the small
intestine.
Several factors affect emptying of the stomach, including the kind of food and the degree of muscle action of
the emptying stomach and the small intestine. Carbohydrates, for example, spend the least amount of time in the
stomach, while protein stays in the stomach longer, and fats the longest. As the food dissolves into the juices
from the pancreas, liver, and intestine, the contents of the intestine are mixed and pushed forward to allow
further digestion.
Finally, the digested nutrients are absorbed through the intestinal walls and transported throughout the body.
The waste products of this process include undigested parts of the food, known as fiber, and older cells that
have been shed from the mucosa. These materials are pushed into the colon, where they remain until the feces
are expelled by a bowel movement.
Absorption and Transport of Nutrients
Most digested molecules of food, as well as water and minerals, are absorbed through the small intestine. The
mucosa of the small intestine contains many folds that are covered with tiny fingerlike projections called villi.
In turn, the villi are covered with microscopic projections called microvilli. These structures create a vast
surface area through which nutrients can be absorbed. Specialized cells allow absorbed materials to cross the
mucosa into the blood, where they are carried off in the bloodstream to other parts of the body for storage or
further chemical change. This part of the process varies with different types of nutrients.
Digestive System
The Mouth Starts Everything Moving
Your digestive (say: dye-jes-tiv) system started working even before you took the first bite of your pizza. And
the digestive system will be busy at work on your chewed-up lunch for the next few hours — or sometimes
days, depending upon what you've eaten. This process, called digestion, allows your body to get the nutrients
and energy it needs from the food you eat. So let's find out what's happening to that pizza, orange, and milk.
Even before you eat, when you smell a tasty food, see it, or think about it, digestion begins. Saliva (say: suhlye-vuh), or spit, begins to form in your mouth.
When you do eat, the saliva breaks down the chemicals in the food a bit, which helps make the food mushy and
easy to swallow. Your tongue helps out, pushing the food around while you chew with your teeth. When you're
ready to swallow, the tongue pushes a tiny bit of mushed-up food called a bolus (say: bow-lus) toward the back
of your throat and into the opening of your esophagus, the second part of the digestive tract.
On the Way Down
The esophagus (say: ih-sof-eh-gess) is like a stretchy pipe that's about 10 inches (25 centimeters) long. It
moves food from the back of your throat to your stomach. But also at the back of your throat is your windpipe,
which allows air to come in and out of your body. When you swallow a small ball of mushed-up food or liquids,
a special flap called the epiglottis (say: ep-ih-glot-iss) flops down over the opening of your windpipe to make
sure the food enters the esophagus and not the windpipe.
If you've ever drunk something too fast, started to cough, and heard someone say that your drink "went down
the wrong way," the person meant that it went down your windpipe by mistake. This happens when the
epiglottis doesn't have enough time to flop down, and you cough involuntarily (without thinking about it) to
clear your windpipe.
Once food has entered the esophagus, it doesn't just drop right into your stomach. Instead, muscles in the walls
of the esophagus move in a wavy way to slowly squeeze the food through the esophagus. This takes about 2 or
3 seconds.
See You in the Stomach
Your stomach, which is attached to the end of the esophagus, is a stretchy sack shaped like the letter J. It has
three important jobs:
1. to store the food you've eaten
2. to break down the food into a liquidy mixture
3. to slowly empty that liquidy mixture into the small intestine
The stomach is like a mixer, churning and mashing together all the small balls of food that came down the
esophagus into smaller and smaller pieces. It does this with help from the strong muscles in the walls of the
stomach and gastric (say: gas-trik) juices that also come from the stomach's walls. In addition to breaking down
food, gastric juices also help kill bacteria that might be in the eaten food.
Onward to the small intestine!
22 Feet Isn't Small at All
The small intestine (say: in-tes-tin) is a long tube that's about 1½ inches to 2 inches (about 3.5 to 5 centimeters)
around, and it's packed inside you beneath your stomach. If you stretched out an adult's small intestine, it would
be about 22 feet long (6.7 meters) — that's like 22 notebooks lined up end to end, all in a row!
The small intestine breaks down the food mixture even more so your body can absorb all the vitamins, minerals,
proteins, carbohydrates, and fats. The grilled chicken on your pizza is full of proteins — and a little fat — and
the small intestine can help extract them with a little help from three friends: the pancreas (say: pan-kree-us),
liver, and gallbladder.
Those organs send different juices to the first part of the small intestine. These juices help to digest food and
allow the body to absorb nutrients. The pancreas makes juices that help the body digest fats and protein. A juice
from the liver called bile helps to absorb fats into the bloodstream. And the gallbladder serves as a warehouse
for bile, storing it until the body needs it.
Your food may spend as long as 4 hours in the small intestine and will become a very thin, watery mixture. It's
time well spent because, at the end of the journey, the nutrients from your pizza, orange, and milk can pass from
the intestine into the blood. Once in the blood, your body is closer to benefiting from the complex carbohydrates
in the pizza crust, the vitamin C in your orange, the protein in the chicken, and the calcium in your milk.
Next stop for these nutrients: the liver! And the leftover waste — parts of the food that your body can't use —
goes on to the large intestine.
Love Your Liver
The nutrient-rich blood comes directly to the liver for processing. The liver filters out harmful substances or
wastes, turning some of the waste into more bile. The liver even helps figure out how many nutrients will go to
the rest of the body, and how many will stay behind in storage. For example, the liver stores certain vitamins
and a type of sugar your body uses for energy.
That's One Large Intestine
At 3 or 4 inches around (about 7 to 10 centimeters), the large intestine is fatter than the small intestine and it's
almost the last stop on the digestive tract. Like the small intestine, it is packed into the body, and would
measure 5 feet (about 1.5 meters) long if you spread it out.
The large intestine has a tiny tube with a closed end coming off it called the appendix (say: uh-pen-dix). It's
part of the digestive tract, but it doesn't seem to do anything, though it can cause big problems because it
sometimes gets infected and needs to be removed.
Like we mentioned, after most of the nutrients are removed from the food mixture there is waste left over —
stuff your body can't use. This stuff needs to be passed out of the body. Can you guess where it ends up? Well,
here's a hint: It goes out with a flush.
Before it goes, it passes through the part of the large intestine called the colon (say: co-lun), which is where the
body gets its last chance to absorb the water and some minerals into the blood. As the water leaves the waste
product, what's left gets harder and harder as it keeps moving along, until it becomes a solid. Yep, it's poop
(also called stool or a bowel movement).
The large intestine pushes the poop into the rectum (say: rek-tum), the very last stop on the digestive tract. The
solid waste stays here until you are ready to go to the bathroom. When you go to the bathroom, you are getting
rid of this solid waste by pushing it through the anus (say: ay-nus). There's the flush we were talking about!
Dig That Digestive System
You can help your digestive system by drinking water and eating a healthy diet that includes foods rich in fiber.
High-fiber foods, like fruits, vegetables, and whole grains, make it easier for poop to pass through your system.
The digestive system is a pretty important part of your body. Without it, you couldn't get the nutrients you need
to grow properly and stay healthy. And next time you sit down to lunch, you'll know where your food goes —
from start to finish!
Muscular System
Did you know you have more than 600 muscles in your body? They do everything from pumping blood
throughout your body to helping you lift your heavy backpack. You control some of your muscles, while others
— like your heart — do their jobs without you thinking about them at all.
Muscles are all made of the same material, a type of elastic tissue (sort of like the material in a rubber band).
Thousands, or even tens of thousands, of small fibers make up each muscle.
You have three different types of muscles in your body: smooth muscle, cardiac (say: KAR-dee-ak) muscle,
and skeletal (say: SKEL-uh-tul) muscle.
Smooth Muscles
Smooth muscles — sometimes also called involuntary muscles — are usually in sheets, or layers, with one layer
of muscle behind the other. You can't control this type of muscle. Your brain and body tell these muscles what
to do without you even thinking about it. You can't use your smooth muscles to make a muscle in your arm or
jump into the air.
But smooth muscles are at work all over your body. In your stomach and digestive system, they contract
(tighten up) and relax to allow food to make its journey through the body. Your smooth muscles come in handy
if you're sick and you need to throw up. The muscles push the food back out of the stomach so it comes up
through the esophagus (say: ih-SAH-fuh-gus) and out of the mouth.
Smooth muscles are also found in your bladder. When they're relaxed, they allow you to hold in urine (pee)
until you can get to the bathroom. Then they contract so that you can push the urine out. These muscles are also
in a woman's uterus, which is where a baby develops. There they help to push the baby out of the mother's body
when it's time to be born.
You'll find smooth muscles at work behind the scenes in your eyes, too. These muscles keep the eyes focused.
A Hearty Muscle
The muscle that makes up the heart is called cardiac muscle. It is also known as the myocardium (say: my-uhKAR-dee-um). The thick muscles of the heart contract to pump blood out and then relax to let blood back in
after it has circulated through the body.
Just like smooth muscle, cardiac muscle works all by itself with no help from you. A special group of cells
within the heart are known as the pacemaker of the heart because it controls the heartbeat.
Skeletal Muscle
Now, let's talk about the kind of muscle you think of when we say
"muscle" — the ones that show how strong you are and let you boot a soccer ball into the goal. These are your
skeletal muscles — sometimes called striated (say: STRY-ay-tud) muscle because the light and dark parts of
the muscle fibers make them look striped (striated is a fancy word meaning striped).
Skeletal muscles are voluntary muscles, which means you can control what they do. Your leg won't bend to
kick the soccer ball unless you want it to. These muscles help to make up the musculoskeletal (say: mus-kyuhlow-SKEL-uh-tul) system — the combination of your muscles and your skeleton, or bones.
Together, the skeletal muscles work with your bones to give your body power and strength. In most cases, a
skeletal muscle is attached to one end of a bone. It stretches all the way across a joint (the place where two
bones meet) and then attaches again to another bone.
Skeletal muscles are held to the bones with the help of tendons (say: TEN-dunz). Tendons are cords made of
tough tissue, and they work as special connector pieces between bone and muscle. The tendons are attached so
well that when you contract one of your muscles, the tendon and bone move along with it.
Skeletal muscles come in many different sizes and shapes to allow them to do many types of jobs. Some of your
biggest and most powerful muscles are in your back, near your spine. These muscles help keep you upright and
standing tall.
They also give your body the power it needs to lift and push things. Muscles in your neck and the top part of
your back aren't as large, but they are capable of some pretty amazing things: Try rotating your head around,
back and forth, and up and down to feel the power of the muscles in your neck. These muscles also hold your
head high.
Face Muscles
You may not think of it as a muscular body part, but your face has plenty of muscles. You can check them out
next time you look in the mirror. Facial muscles don't all attach directly to bone like they do in the rest of the
body. Instead, many of them attach under the skin. This allows you to contract your facial muscles just a tiny bit
and make dozens of different kinds of faces. Even the smallest movement can turn a smile into a frown. You
can raise your eyebrow to look surprised or wiggle your nose.
And while you're looking at your face, don't pass over your tongue — a muscle that's attached only at one end!
Your tongue is actually made of a group of muscles that work together to allow you to talk and help you chew
food. Stick out your tongue and wiggle it around to see those muscles at work.
Major Muscles
Because there are so many skeletal muscles in your body, we can't list them all here. But here are a few of the
major ones:
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In each of your shoulders is a deltoid (say: DEL-toyd) muscle. Your deltoid muscles help you move
your shoulders every which way — from swinging a softball bat to shrugging your shoulders when
you're not sure of an answer.
The pectoralis (say: pek-tuh-RAH-lus) muscles are found on each side of your upper chest. These are
usually called pectorals (say: PEK-tuh-rulz), or pecs, for short. When many boys hit puberty, their
pectoral muscles become larger. Many athletes and bodybuilders have large pecs, too.
Below these pectorals, down under your ribcage, are your rectus abdominus (say: REK-tus ab-DAHMuh-nus) muscles, or abdominals (say: ab-DAHM-uh-nulz). They're often called abs for short.
When you make a muscle in your arm, you tense your biceps (say: BYE-seps) muscle. When you
contract your biceps muscle, you can actually see it push up under your skin.
Your quadriceps (say: KWAD-ruh-seps), or quads, are the muscles on the front of your thighs. Many
people who run, bike, or play sports develop large, strong quads.
And when it's time for you to take a seat? You'll be sitting on your gluteus maximus (say: GLOOT-eeus MAK-suh-mus), the muscle that's under the skin and fat in your behind!
Skeletal System
How Bones Grow
When you were a baby, you had tiny hands, tiny feet, and tiny everything! Slowly, as you grew older,
everything became a bit bigger, including your bones.
A baby's body has about 300 bones at birth. These eventually fuse (grow together) to form the 206 bones that
adults have. Some of a baby's bones are made entirely of a special material called cartilage (say: KAR-tel-ij).
Other bones in a baby are partly made of cartilage. This cartilage is soft and flexible. During childhood, as you
are growing, the cartilage grows and is slowly replaced by bone, with help from calcium.
By the time you are about 25, this process will be complete. After this happens, there can be no more growth —
the bones are as big as they will ever be. All of these bones make up a skeleton that is both very strong and very
light.
Your Spine
Your spine is one part of the skeleton that's easy to check out: Reach around to the center of your back and
you'll feel its bumps under your fingers.
The spine lets you twist and bend, and it holds your body upright. It also protects the spinal cord, a large bundle
of nerves that sends information from your brain to the rest of your body. The spine is special because it isn't
made of one or even two bones: It's made of 26 bones in all! These bones are called vertebrae (say: VER-tuhbray) and each one is shaped like a ring.
There are different types of vertebrae in the spine and each does a different kind of job:
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The first seven vertebrae at the top are called the cervical (say: SIR-vih-kul) vertebrae. These bones are
in the back of your neck, just below your brain, and they support your head and neck. Your head is
pretty heavy, so it's lucky to have help from the cervical vertebrae!
Below the cervical vertebrae are the thoracic (say: thuh-RAS-ick) vertebrae, and there are 12 in all.
These guys anchor your ribs in place. Below the thoracic vertebrae are five lumbar (say: LUM-bar)
vertebrae. Beneath the lumbar vertebrae is the sacrum (say: SAY-krum), which is made up of five
vertebrae that are fused together to form one single bone.
Finally, all the way at the bottom of the spine is the coccyx (say: COK-siks), which is one bone made of
four fused vertebrae. The bottom sections of the spine are important when it comes to bearing weight
and giving you a good center of gravity. So when you pick up a heavy backpack, the lumbar vertebrae,
sacrum, and coccyx give you the power. When you dance, skip, and even walk, these parts help keep
you balanced.
In between each vertebra (the name for just one of the vertebrae) are small disks made of cartilage. These disks
keep the vertebrae from rubbing against one another, and they also act as your spine's natural shock absorbers.
When you jump in the air, or twist while slamming a dunk, the disks give your vertebrae the cushioning they
need.
Your Ribs
Your heart, lungs, and liver are all very important, and luckily you've got ribs to keep
them safe. Ribs act like a cage of bones around your chest. It's easy to feel the bottom of this cage by running
your fingers along the sides and front of your body, a few inches below your heart. If you breathe in deeply, you
can easily feel your ribs right in the front of your body, too. Some thin kids can even see a few of their ribs right
through their skin.
Your ribs come in pairs, and the left and right sides of each pair are exactly the same. Most people have 12 pairs
of ribs, but some people are born with one or more extra ribs, and some people might have one pair less.
All 12 pairs of ribs attach in the back to the spine, where they are held in place by the thoracic vertebrae. The
first seven pairs of ribs attach in the front to the sternum (say: STUR-num), a strong bone in the center of your
chest that holds those ribs in place. The remaining sets of ribs don't attach to the sternum directly. The next
three pairs are held on with cartilage to the ribs above them.
The very last two sets of ribs are called floating ribs because they aren't connected to the sternum or the ribs
above them. But don't worry, these ribs can't ever float away. Like the rest of the ribs, they are securely attached
to the spine in the back.
Your Joints
The place where two bones meet is called a joint. Some joints move and others don't.
Fixed joints are fixed in place and don't move at all. Your skull has some of these joints (called sutures,
remember?), which close up the bones of the skull in a young person's head. One of these joints is called the
parieto-temporal (say: par-EYE-ih-toh TEM-puh-rul) suture — it's the one that runs along the side of the skull.
Moving joints are the ones that let you ride your bike, eat cereal, and play a video game — the ones that allow
you to twist, bend, and move different parts of your body. Some moving joints, like the ones in your spine,
move only a little. Other joints move a lot. One of the main types of moving joints is called a hinge joint. Your
elbows and knees each have hinge joints, which let you bend and then straighten your arms and legs. These
joints are like the hinges on a door. Just as most doors can only open one way, you can only bend your arms and
legs in one direction. You also have many smaller hinge joints in your fingers and toes.
Another important type of moving joint is the ball and socket joint. You can find these joints at your shoulders
and hips. They are made up of the round end of one bone fitting into a small cup-like area of another bone. Ball
and socket joints allow for lots of movement in every direction. Make sure you've got lots of room, and try
swinging your arms all over the place.
Have you ever seen someone put oil on a hinge to make it work easier or stop squeaking? Well, your joints
come with their own special fluid called synovial fluid (say: SIH-no-vee-ul) that helps them move freely.
Bones are held together at the joints by ligaments (say: LIH-guh-mints), which are like very strong rubber
bands.
Excretory System
Pee is one of the first body fluids a kid learns about. You probably learned about pee (also called urine) when
you were little and started using the toilet instead of diapers. Now that you're older, you can understand much
more about the amazing yellow stuff called pee.
Parts of the Urinary Tract
You drink, you pee. But urine is more than just that drink you had a few hours ago. The body produces pee as a
way to get rid of waste and extra water that it doesn't need. Before leaving your body, urine travels through the
urinary tract.
The urinary tract is a pathway that includes the:
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kidneys: two bean-shaped organs that filter waste from the blood and produce urine
ureters: two thin tubes that take pee from the kidney to the bladder
bladder: a sac that holds pee until it's time to go to the bathroom
urethra: the tube that carries urine from the bladder out of the body when you pee
The kidneys are key players in the urinary tract. They do two important jobs — filter waste from the blood and
produce pee to get rid of it. If they didn't do this, toxins (bad stuff) would quickly build up in your body and
make you sick. That's why you hear about people getting kidney transplants sometimes. You need at least one
working kidney to be healthy.
You might wonder how your body ends up with waste it needs to get rid of. Body processes such as digestion
and metabolism (when the body turns food into energy) produce wastes, or byproducts. The body takes what it
needs, but the waste has to go somewhere. Thanks to the kidneys and pee, it has a way to get out.
When you're asked to give a urine sample during a doctor's visit, the results reveal how well your two kidneys
are working. For example, white blood cells in the urine can be a sign of an infection.
Pee also is a way for your body to keep the right amount of water. Did you ever notice that if you drink a lot,
you pee more and the pee is pale yellow? That's because your body is getting rid of extra water and your pee has
more water in it than usual.
What's Pee Made Of?
Let's talk more about how the kidneys filter blood. When blood goes through the kidneys, water and some of the
other stuff that is in blood (like protein, glucose, and other nutrients) go back into the bloodstream, while the
excess stuff and waste is taken out. Urine is what is left behind. But what is it exactly?
Urine contains:
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water
urea, a waste product that forms when proteins are broken down
urochrome, a pigmented blood product that gives urine its yellowish color
salts
creatinine, a waste product that forms with the normal breakdown of muscle
byproducts of bile from the liver
ammonia
You've Got to Go!
Once pee is produced, it travels from the kidney to the bladder, where it's stored until you need to go to the
bathroom. The bladder expands as it fills; when it's full, nerve endings in the bladder wall send a message to the
brain that you need to pee.
When you're in the bathroom, ready to go, the bladder walls contract and the sphincter (a ringlike muscle that
guards the exit from the bladder to the urethra) relaxes. The urine then flows from the bladder and out of the
body through the urethra. For boys, the urethra ends at the tip of the penis. For girls, it's above the vaginal
opening.
Urinary Tract Health
You might not think much about peeing or your urinary tract, but here's how you can help keep everything
flowing as it should:
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Drink enough fluids. There's no magic amount, but be sure to drink plenty of water, especially when it's
warm out or you're exercising and playing.
For girls: Wipe from front to back after going poop. Because of where the urethra is for girls, it's easy
for bacteria from poop to get in that area. If some of those bacteria end up in the urinary tract, you could
get an infection known as a UTI (urinary tract infection).
For everyone: Go to the bathroom when you need to go. Holding too long isn't good for your urinary
tract — and it can lead to accidents. Uh-oh!
Kidneys
Kidneys normally come in pairs. If you've ever seen a kidney bean, then you have a pretty good idea what the
kidneys look like. Each kidney is about 5 inches (about 13 centimeters) long and about 3 inches (about 8
centimeters) wide — about the size of a computer mouse.
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To locate your kidneys, put your hands on your hips, then slide your hands up until you can feel your
ribs. Now if you put your thumbs on your back, you will know where your kidneys are. You can't feel
them, but they are there. Read on to find out more about the cool kidneys.
Cleaning Up
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One of the main jobs of the kidneys is to filter the waste out of the blood. How does the waste get in
your blood? Well, your blood delivers nutrients to your body. Chemical reactions occur in the cells of
your body to break down the nutrients. Some of the waste is the result of these chemical reactions. Some
is just stuff your body doesn't need because it already has enough. The waste has to go somewhere; this
is where the kidneys come in.
The Path of Pee
The waste that is collected combines with water (which is also filtered out of the kidneys) to make urine (pee).
As each kidney makes urine, the urine slides down a long tube called the ureter (say: yu-REE-ter) and collects
in the bladder, a storage sac that holds the urine. When the bladder is about halfway full, your body tells you to
go to the bathroom. When you pee, the urine goes from the bladder down another tube called the urethra (say:
yu-REE-thruh) and out of your body.
The kidneys, the bladder, and their tubes are called the urinary system. Here's a list of all of the parts of the
urinary system:
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the kidneys: filters that take the waste out of the blood and make urine
the ureters: tubes that carry the urine from each kidney to the bladder
the bladder: a bag that collects the urine
the urethra: a tube that carries the urine from the bladder out of the body
Keeping a Balance
The kidneys also balance the volume of fluids and minerals in the body. This balance in the body is called
homeostasis (say: HOH-mee-oh-STAY-sus).
If you put all of the water that you take in on one side of a scale and all of the water your body gets rid of on the
other side of a scale, the sides of the scale would be balanced. Your body gets water when you drink it or other
liquids. You also get water from some foods, like fruits and vegetables.
Water leaves your body in several ways. It comes out of your skin when you sweat, out of your mouth when
you breathe, and out of your urethra in urine when you go to the bathroom. There is also water in your bowel
movements (poop).
When you feel thirsty, your brain is telling you to get more fluids to keep your body as balanced as possible. If
you don't have enough fluids in your body, the brain communicates with the kidneys by sending out a hormone
that tells the kidneys to hold on to some fluids. When you drink more, this hormone level goes down, and the
kidneys will let go of more fluids.
You might notice that sometimes your urine is darker in color than other times. Remember, urine is made up of
water plus the waste that is filtered out of the blood. If you don't take in a lot of fluids or if you're exercising and
sweating a lot, your urine has less water in it and it appears darker. If you're drinking lots of fluids, the extra
fluid comes out in your urine, and it will be lighter.
What Else Do Kidneys Do?
Kidneys are always busy. Besides filtering the blood and balancing fluids every second during the day, the
kidneys constantly react to hormones that the brain sends them. Kidneys even make some of their own
hormones. For example, the kidneys produce a hormone that tells the body to make red blood cells.
Now you know what the kidneys do and how important they are. Maybe next Valentine's Day, instead of the
same old heart, you can give your parents a special card featuring the kidneys!
Respiratory System
Your lungs make up one of the largest organs in your body, and they work with your respiratory system to
allow you to take in fresh air, get rid of stale air, and even talk. Let's take a tour of the lungs! Locate Those
Lungs
Your lungs are in your chest, and they are so large that they take up most of the space in there. You have two
lungs, but they aren't the same size the way your eyes or nostrils are. Instead, the lung on the left side of your
body is a bit smaller than the lung on the right. This extra space on the left leaves room for your heart.
Your lungs are protected by your rib cage, which is made up of 12 sets of ribs. These ribs are connected to your
spine in your back and go around your lungs to keep them safe. Beneath the lungs is the diaphragm (say: DYuh-fram), a dome-shaped muscle that works with your lungs to allow you to inhale (breathe in) and exhale
(breathe out) air.
You can't see your lungs, but it's easy to feel them in action: Put your hands on your chest and breathe in very
deeply. You will feel your chest getting slightly bigger. Now breathe out the air, and feel your chest return to its
regular size. You've just felt the power of your lungs!
A Look Inside the Lungs
From the outside, lungs are pink and a bit squishy, like a sponge. But the inside contains the real lowdown on
the lungs! At the bottom of the trachea (say: TRAY-kee-uh), or windpipe, there are two large tubes. These
tubes are called the main stem bronchi (say: BRONG-kye), and one heads left into the left lung, while the other
heads right into the right lung.
Each main stem bronchus (say: BRONG-kuss) — the name for just one of the bronchi — then branches off into
tubes, or bronchi, that get smaller and even smaller still, like branches on a big tree. The tiniest tubes are called
bronchioles (say: BRONG-kee-oles), and there are about 30,000 of them in each lung. Each bronchiole is about
the same thickness as a hair.
At the end of each bronchiole is a special area that leads into clumps of teeny tiny air sacs called alveoli (say:
al-VEE-oh-lie). There are about 600 million alveoli in your lungs and if you stretched them out, they would
cover an entire tennis court. Now that's a load of alveoli! Each alveolus (say: al-VEE-oh-luss) — what we call
just one of the alveoli — has a mesh-like covering of very small blood vessels called capillaries (say: CAP-ill-
er-ees). These capillaries are so tiny that the cells in your blood need to line up single file just to march through
them.
All About Inhaling
When you're walking your dog, cleaning your room, or spiking a volleyball, you probably don't think about
inhaling (breathing in) — you've got other things on your mind! But every time you inhale air, dozens of body
parts work together to help get that air in there without you ever thinking about it.
As you breathe in, your diaphragm contracts and flattens out. This allows it to move down, so your lungs have
more room to grow larger as they fill up with air. "Move over, diaphragm, I'm filling up!" is what your lungs
would say. And the diaphragm isn't the only part that gives your lungs the room they need. Your rib muscles
also lift the ribs up and outward to give the lungs more space.
At the same time, you inhale air through your mouth and nose, and the air heads down your trachea, or
windpipe. On the way down the windpipe, tiny hairs called cilia (say: SILL-ee-uh) move gently to keep mucus
and dirt out of the lungs. The air then goes through the series of branches in your lungs, through the bronchi and
the bronchioles.
Thank You, Alveoli!
The air finally ends up in the 600 million alveoli. As these millions of alveoli fill up with air, the lungs get
bigger. Remember that experiment where you felt your lungs get larger? Well, you were really feeling the
power of those awesome alveoli!
It's the alveoli that allow oxygen from the air to pass into your blood. All the cells in the body need oxygen
every minute of the day. Oxygen passes through the walls of each alveolus into the tiny capillaries that surround
it. The oxygen enters the blood in the tiny capillaries, hitching a ride on red blood cells and traveling through
layers of blood vessels to the heart. The heart then sends the oxygenated (filled with oxygen) blood out to all the
cells in the body.
Waiting to Exhale
When it's time to exhale (breathe out), everything happens in reverse: Now it's the diaphragm's turn to say,
"Move it!" Your diaphragm relaxes and moves up, pushing air out of the lungs. Your rib muscles become
relaxed, and your ribs move in again, creating a smaller space in your chest.
By now your cells have used the oxygen they need, and your blood is carrying carbon dioxide and other wastes
that must leave your body. The blood comes back through the capillaries and the wastes enter the alveoli. Then
you breathe them out in the reverse order of how they came in — the air goes through the bronchioles, out the
bronchi, out the trachea, and finally out through your mouth and nose.
The air that you breathe out not only contains wastes and carbon dioxide, but it's warm, too! As air travels
through your body, it picks up heat along the way. You can feel this heat by putting your hand in front of your
mouth or nose as you breathe out. What is the temperature of the air that comes out of your mouth or nose?
With all this movement, you might be wondering why things don't get stuck as the lungs fill and empty!
Luckily, your lungs are covered by two really slick special layers called pleural (say: PLOO-ral) membranes.
These membranes are separated by a fluid that allows them to slide around easily while you inhale and exhale.
Time for Talk
Your lungs are important for breathing . . . and also for talking! Above the trachea (windpipe) is the larynx
(say: LAIR-inks), which is sometimes called the voice box. Across the voice box are two tiny ridges called
vocal cords, which open and close to make sounds. When you exhale air from the lungs, it comes through the
trachea and larynx and reaches the vocal cords. If the vocal cords are closed and the air flows between them, the
vocal cords vibrate and a sound is made.
The amount of air you blow out from your lungs determines how loud a sound will be and how long you can
make the sound. Try inhaling very deeply and saying the names of all the kids in your class — how far can you
get without taking the next breath? The next time you're outside, try shouting and see what happens — shouting
requires lots of air, so you'll need to breathe in more frequently than you would if you were only saying the
words.
Experiment with different sounds and the air it takes to make them — when you giggle, you let out your breath
in short bits, but when you burp, you let swallowed air in your stomach out in one long one! When you hiccup,
it's because the diaphragm moves in a funny way that causes you to breathe in air suddenly, and that air hits
your vocal cords when you're not ready.
Love Your Lungs
Your lungs are amazing. They allow you to breathe, talk to your friend, shout at a game, sing, laugh, cry, and
more! And speaking of a game, your lungs even work with your brain to help you inhale and exhale a larger
amount of air at a more rapid rate when you're running a mile — all without you even thinking about it once.
Keeping your lungs looking and feeling healthy is a smart idea, and the best way to keep your lungs pink and
healthy is not to smoke. Smoking isn't good for any part of your body, and your lungs especially hate it.
Cigarette smoke damages the cilia in the trachea so they can no longer move to keep dirt and other substances
out of the lungs. Your alveoli get hurt too, because the chemicals in cigarette smoke can cause the walls of the
delicate alveoli to break down, making it much harder to breathe.
Finally, cigarette smoke can damage the cells of the lungs so much that the healthy cells go away, only to be
replaced by cancer cells. Lungs are normally tough and strong, but when it comes to cigarettes, they can be hurt
easily — and it's often very difficult or impossible to make them better. If you need to work with chemicals in
an art or shop class, be sure to wear a protective mask to keep chemical fumes from entering your lungs.
You can also show your love for your lungs by exercising! Exercise is good for every part of your body, and
especially for your lungs and heart. When you take part in vigorous exercise (like biking, running, or
swimming, for example), your lungs require more air to give your cells the extra oxygen they need. As you
breathe more deeply and take in more air, your lungs become stronger and better at supplying your body with
the air it needs to succeed. Keep your lungs healthy and they will thank you for life!
Circulatory System
Working That Muscle
Your heart is really a muscle. It's located a little to the left of the middle of your
chest, and it's about the size of your fist. There are lots of muscles all over your body — in your arms, in your
legs, in your back, even in your behind.
But the heart muscle is special because of what it does. The heart sends blood around your body. The blood
provides your body with the oxygen and nutrients it needs. It also carries away waste.
Your heart is sort of like a pump, or two pumps in one. The right side of your heart receives blood from the
body and pumps it to the lungs. The left side of the heart does the exact opposite: It receives blood from the
lungs and pumps it out to the body.
We Got the Beat
How does the heart beat? Before each beat, your heart fills with blood. Then its muscle contracts to squirt the
blood along. When the heart contracts, it squeezes — try squeezing your hand into a fist. That's sort of like what
your heart does so it can squirt out the blood. Your heart does this all day and all night, all the time. The heart is
one hard worker!
It's Great to Circulate
You probably guessed that the blood just doesn't slosh around your body once it leaves the heart. It moves
through many tubes called arteries and veins, which together are called blood vessels. These blood vessels are
attached to the heart. The blood vessels that carry blood away from the heart are called arteries. The ones that
carry blood back to the heart are called veins.
The movement of the blood through the heart and around the body is called circulation (say: sur-kyoo-layshun), and your heart is really good at it — it takes less than 60 seconds to pump blood to every cell in your
body.
Your body needs this steady supply of blood to keep it working right. Blood delivers oxygen to all the body's
cells. To stay alive, a person needs healthy, living cells. Without oxygen, these cells would die. If that oxygenrich blood doesn't circulate as it should, a person could die.
The left side of your heart sends that oxygen-rich blood out to the body. The body takes the oxygen out of the
blood and uses it in your body's cells. When the cells use the oxygen, they make carbon dioxide and other stuff
that gets carried away by the blood. It's like the blood delivers lunch to the cells and then has to pick up the
trash!
The returning blood enters the right side of the heart. The right ventricle pumps the blood to the lungs for a little
freshening up. In the lungs, carbon dioxide is removed from the blood and sent out of the body when we exhale.
What's next? An inhale, of course, and a fresh breath of oxygen that can enter the blood to start the process
again. And remember, it all happens in about a minute!
Pretty Cool — It's My Pulse!
Even though your heart is inside you, there is a cool way to know it's working from the outside. It's your pulse.
You can find your pulse by lightly pressing on the skin anywhere there's a large artery running just beneath your
skin. Two good places to find it are on the side of your neck and the inside of your wrist, just below the thumb.
You'll know that you've found your pulse when you can feel a small beat under your skin. Each beat is caused
by the contraction (squeezing) of your heart. If you want to find out what your heart rate is, use a watch with a
second hand and count how many beats you feel in 1 minute. When you are resting, you will probably feel
between 70 and 100 beats per minute.
When you run around a lot, your body needs a lot more oxygen-filled blood. Your heart pumps faster to supply
the oxygen-filled blood that your body needs. You may even feel your heart pounding in your chest. Try
running in place or jumping rope for a few minutes and taking your pulse again — now how many beats do you
count in 1 minute?
Endocrine System
You might say endocrine (say: en-doh-krin) glands are a little bossy — they tell your cells what to do! But
that's actually a good thing. Without your endocrine glands — and the hormones they release — your cells
wouldn't know when to do important things.
For instance, your bones wouldn't get the message that it's time for you to grow and get bigger. And your body
wouldn't know that it's time to begin puberty, the body changes that turn kids into grownups.
You have a variety of endocrine glands in different sizes and shapes located in different parts of the body. You
might be surprised to learn that the pituitary (say: pih-too-uh-ter-ee) gland, which is about the size of a pea, is
the "master gland" of the endocrine system. It makes and releases a bunch of hormones that control other glands
and body functions. Tiny and tucked beneath your brain, the pituitary helps you grow big by producing growth
hormone.
Your thyroid (say: thy-royd) gland is in your neck and it's shaped like a bowtie or a butterfly. It makes
hormones that are important for growth and it helps you stay alert and full of energy.
Your adrenal (say: uh-dree-nul) glands are really important to your body in times of trouble, like when you're
sick or under stress. Adrenaline (say: uh-dreh-nuh-lin), one of the adrenal gland hormones, gives you the boost
you need if you're being chased by a wild animal — or even your brother!
Insulin Is Essential
Your pancreas (say: pan-kree-us) is your largest endocrine gland and it's found in your belly. The pancreas
makes several hormones, including insulin (say: in-suh-lin), which helps glucose (say: gloo-kose), the sugar
that's in your blood, enter the cells of your body. Your cells need to be fueled with glucose to function, like a
car's engine needs gas. And we all know what happens when you run out of fuel!
Your body does an amazing job of making sure that hormones are released in just the right amounts at just the
right time. If there's a problem with the endocrine system, a person's body might not grow like it should or it
might not work the way it's supposed to.
Diabetes (say: dye-uh-be-tees) is one common problem with the endocrine system. It occurs when a person's
pancreas doesn't make enough insulin. It's also an endocrine problem if a kid isn't growing as quickly as
expected because his or her pituitary gland isn't making enough growth hormone.
Fortunately, special doctors called endocrinologists (say: en-doh-krin-all-oh-jistz) know a lot about the
endocrine system and can help treat people with hormone problems. But most kids will never need to worry
about their endocrine system because it works fine on its own. How does that make a kid feel? Gland-tastic!
What are glands?
Glands release hormones (say: hor-mones), which are substances inside your body that tell it how to work and how to
grow. Glands that do this are part of the endocrine (say: en-doh-krin) system. Puberty — body changes that turn a kid
into an adult — depend on the endocrine system.
Hormones
Hormones are special chemicals your body makes to help it do certain things - like grow up! Hormones are
important when you start to go through puberty, which is when you begin developing into an adult. During this
time, you're loaded with hormones that tell your body that it's time to start changing.
Pancreas
The pancreas is a long, flat gland in your belly. It sits behind the stomach and produces enzymes that are
important for digestion. Insulin and glucagon, which help control the level of glucose (a type of sugar) in the
blood, are also made in the pancreas.
Insulin
If someone has diabetes, he or she has trouble with a hormone called insulin. Insulin, which is made in the
pancreas, lowers the level of glucose (a type of sugar) in the blood. It does this by helping glucose enter the
body's cells. Glucose is the main source of energy for the cells. And because you're made up of cells, you want
those cells to get the fuel they need!
Your body gets glucose from the food you eat, and it travels through the bloodstream. But without insulin,
glucose can't get into the cells. In diabetes, the pancreas doesn't make enough insulin or the body can't respond
normally to the insulin that is made. This causes the glucose level in the blood to rise.
Blood Tissue
You know what blood is — it's that red stuff that oozes out if you get a paper cut. The average person has about
1 to 1½ gallons (4-6 liters) of it. But what is blood, really, and where does it come from?
How Does the Body Make Blood?
It's not made in a kitchen, but blood has ingredients, just like a recipe. To make blood, your body needs to mix:




red blood cells, which carry oxygen throughout the body
white blood cells, which fight infections
platelets, which are cells that help you stop bleeding if you get a cut
plasma, a yellowish liquid that carries nutrients, hormones, and proteins throughout the body
Your body doesn't go to the store to buy those ingredients. It makes them. Bone marrow — that goopy stuff
inside your bones — makes the red blood cells, the white blood cells, and the platelets. Plasma is mostly water,
which is absorbed from the intestines from what you drink and eat, with the liver supplying important proteins.
Put all these ingredients together and you have blood — an essential part of the circulatory system. Thanks to
your heart (which pumps blood) and your blood vessels (which carry it), blood travels throughout your body
from your head to your toes.
Let's find out more about each ingredient.
Red Blood Cells
Red blood cells (also called erythrocytes, say: ih-rith-ruh-sytes) look like flattened basketballs. Most of the
cells in the blood are red blood cells. They carry around an important chemical called hemoglobin (say: heemuh-glow-bin) that gives blood its red color.
Blood and breathing go hand in hand. How? The hemoglobin in blood delivers oxygen, which you get from the
air you breathe, to all parts of your body. Without oxygen, your body couldn't keep working and stay alive.
White Blood Cells
White blood cells (also called leukocytes, say: loo-kuh-sytes) are bigger than red blood cells. There are usually
not a whole lot of white blood cells floating around in your blood when you're healthy. Once you get sick,
though, your body makes some more to protect you.
There are a couple types of white blood cells that do different things to keep you well:
Granulocytes
You know how your skin gets a little red and swollen around a cut or scrape? That means the granulocytes are
doing their jobs. They have a lot to do with how your body cleans things up and helps wounds heal after an
injury. Granulocytes also help prevent infection by surrounding and destroying things that aren't supposed to be
in your body and by killing germs.
Lymphocytes
There are two types of lymphocytes: B cells and T cells. B cells help make special proteins called antibodies
that recognize stuff that shouldn't be in your body, like bacteria or a virus you get from a sick friend. Antibodies
are very specific, and can recognize only a certain type of germ. Once the antibody finds it, it gets rid of the
germ so it can't hurt you.
The really cool part is that even after you are better, B cells can become memory cells that remember how to
make the special antibody so that if the same germ infects you again, it can kill the germ even faster! T cells
also battle germs that invade the body, but instead of making antibodies, they work by making special
chemicals that help fight the infection.
Monocytes
Monocytes are white blood cells that fight infection by surrounding and destroying bacteria and viruses.
Platelets
Platelets, also called thrombocytes (say: throm-buh-sytes), are tiny round cells that help to make sure you don't
bleed too much once you get a cut or scrape. Cuts and scrapes break blood vessels. If a platelet reaches a blood
vessel that's been broken open, it sends out a chemical signal that makes other nearby platelets start to stick
together inside the vessel.
After the platelets form this plug, they send out more chemical signals that attract clotting factors. These
clotting factors work together to make a web of tiny protein threads. The platelets and this web of protein come
together to make a blood clot. The clot keeps your blood inside the vessel while the break in the blood vessel
heals up. Without platelets, you'd need more than a bandage to catch the blood when you scrape your knee!
Plasma
Plasma is a yellowish liquid that is mostly water. But it also carries important nutrients, hormones, and proteins
throughout the body. Nutrients are chemicals from the food you eat that give your body energy and other things
your body's cells need to do their work and keep you healthy.
Hormones carry messages throughout your body, telling it what to do and when. An example of a hormone is
growth hormone. It gets your bones and muscles to grow. Many proteins in plasma are really important to your
body, like the clotting factors that help you stop bleeding if you get a cut or a scrape.
Plasma also carries away cell waste — chemicals that the cell doesn't want anymore. Nutrients, hormones,
proteins, and waste are dissolved in the plasma — kind of like the cocoa mix that dissolves in a cup of hot
water. What are the marshmallows? The blood cells — they float in the plasma.
Hey, What's Your Type?
Everybody's blood is red, but it's not all the same. There are eight blood types, described using the letters A, B,
and O. Those letters stand for certain proteins found on the red blood cells. Not everyone has the same proteins.
In addition to getting a letter or two, a person's blood is either "positive" or "negative." That doesn't mean one
person's blood is good and another person's blood is bad. It's a way of keeping track of whether someone's blood
has a certain protein called Rh protein. This protein is called "Rh" because scientists found it while studying
Rhesus monkeys. If your blood is positive, you have this protein. If it's negative, you don't. Either way is totally
fine.
People have one of these eight different blood types:
1.
2.
3.
4.
5.
6.
7.
8.
A negative
A positive
B negative
B positive
O negative
O positive
AB negative
AB positive
Blood types are important if a person ever wants to donate blood or needs a blood transfusion. Getting blood of
the wrong type can make a person sick. That's why hospitals and blood banks are very careful with donated
blood and make sure the person gets the right type.
People might need blood transfusions when they're sick or if they lose blood. Without enough healthy blood, the
body won't get the oxygen and energy it needs. Healthy blood also protects you from germs and other invaders.
Now that you know how important blood is, what can you do? Kids generally aren't allowed to donate blood,
but when you're older consider giving the gift of life!
Nervous System
How do you remember the way to your friend's house? Why do your eyes blink without you ever thinking about
it? Where do dreams come from? Your brain is in charge of these things and a lot more.
In fact, your brain is the boss of your body. It runs the show and controls just about everything you do, even
when you're asleep. Not bad for something that looks like a big, wrinkly, gray sponge.
Your brain has many different parts that work together. We're going to talk about these five parts, which are key
players on the brain team:
1.
2.
3.
4.
5.
cerebrum (say: suh-ree-brum)
cerebellum (say: sair-uh-bell-um)
brain stem
pituitary gland (say: puh-too-uh-ter-ee gland)
hypothalamus (say: hy-po-thal-uh-mus)
The Biggest Part: the Cerebrum
The biggest part of the brain is the cerebrum. The cerebrum makes up 85% of the brain's
weight, and it's easy to see why. The cerebrum is the thinking part of the brain and it controls your voluntary
muscles — the ones that move when you want them to. So you can't dance — or kick a soccer ball — without
your cerebrum.
When you're thinking hard, you're using your cerebrum. You need it to solve math problems, figure out a video
game, and draw a picture. Your memory lives in the cerebrum — both short-term memory (what you ate for
dinner last night) and long-term memory (the name of that roller-coaster you rode on two summers ago). The
cerebrum also helps you reason, like when you figure out that you'd better do your homework now because your
mom is taking you to a movie later.
The cerebrum has two halves, with one on either side of the head. Some scientists think that the right half helps
you think about abstract things like music, colors, and shapes. The left half is said to be more analytical, helping
you with math, logic, and speech. Scientists do know for sure that the right half of the cerebrum controls the left
side of your body, and the left half controls the right side.
The Cerebellum's Balancing Act
Next up is the cerebellum. The cerebellum is at the back of the brain, below the
cerebrum. It's a lot smaller than the cerebrum at only 1/8 of its size. But it's a very important part of the brain. It
controls balance, movement, and coordination (how your muscles work together). Because of your cerebellum,
you can stand upright, keep your balance, and move around. Think about a surfer riding the waves on his board.
What does he need most to stay balanced? The best surfboard? The coolest wetsuit? Nope — he needs his
cerebellum!
Brain Stem Keeps You Breathing — and More
Another brain part that's small but mighty is the brain stem. The brain stem sits beneath
the cerebrum and in front of the cerebellum. It connects the rest of the brain to the spinal cord, which runs down
your neck and back. The brain stem is in charge of all the functions your body needs to stay alive, like breathing
air, digesting food, and circulating blood.
Part of the brain stem's job is to control your involuntary muscles — the ones that work automatically, without
you even thinking about it. There are involuntary muscles in the heart and stomach, and it's the brain stem that
tells your heart to pump more blood when you're biking or your stomach to start digesting your lunch. The brain
stem also sorts through the millions of messages that the brain and the rest of the body send back and forth.
Whew! It's a big job being the brain's secretary!
You Have Some Nerve!
So the brain is boss, but it can't do it alone. It needs some nerves — actually a lot of them. And it needs the
spinal cord, which is a long bundle of nerves inside your spinal column, the vertebrae that protect it. It's the
spinal cord and nerves — known as the nervous system — that let messages flow back and forth between the
brain and body.
If a spiky cactus falls off a shelf headed right for your best friend, your nerves and brain communicate so that
you jump up and yell for your friend to get out of the way. If you're really good, maybe you're able to catch the
plant before it hits your friend!
But you might wonder about these nerves, which you can't see without a microscope. What are they anyway?
The nervous system is made up of millions and millions of neurons (say: nur-onz), which are microscopic cells.
Each neuron has tiny branches coming off it that let it connect to many other neurons.
When you were born, your brain came with all the neurons it will ever have, but many of them were not
connected to each other. When you learn things, the messages travel from one neuron to another, over and over.
Eventually, the brain starts to create connections (or pathways) between the neurons, so things become easier
and you can do them better and better.
Think back to the first time you rode a bike. Your brain had to think about pedaling, staying balanced, steering
with the handlebars, watching the road, and maybe even hitting the brakes — all at once. Hard work, right? But
eventually, as you got more practice, the neurons sent messages back and forth until a pathway was created in
your brain. Now you can ride your bike without thinking about it because the neurons have successfully created
a "bike riding" pathway.
Emotion Location
With all the other things it does, is it any surprise that the brain runs your emotions? Maybe you got the exact
toy you wanted for your birthday and you were really happy. Or your friend is sick and you feel sad. Or your
little brother messed up your room, so you're really angry! Where do those feelings come from? Your brain, of
course.
Your brain has a little bunch of cells on each side called the amygdala (say: uh-mig-duh-luh). The word
amygdala is Latin for almond, and that's what this area looks like. Scientists believe that the amygdala is
responsible for emotion. It's normal to feel all different kinds of emotions, good and bad. Sometimes you might
feel a little sad, and other times you might feel scared, or silly, or glad.
Be Good to Your Brain
So what can you do for your brain? Plenty.





Eat healthy foods. They contain potassium and calcium, two minerals that are important for the nervous
system.
Get a lot of playtime (exercise).
Wear a helmet when you ride your bike or play other sports that require head protection.
Don't drink alcohol, take drugs, or use tobacco.
Use your brain by doing challenging activities, such as puzzles, reading, playing music, making art, or
anything else that gives your brain a workout!
Nervous System
Say: ner-vus sis-tem
The nervous system controls everything you do, including breathing, walking, thinking, and feeling. This
system is made up of your brain, spinal cord, and all the nerves of your body. The brain is the control center and
the spinal cord is the major highway to and from the brain. The nerves carry the messages to and from the body,
so the brain can interpret them and take action.
The spinal cord
The spinal cord is along tube like structure which extends from the brain. The spinal cord is composed of a
series of 31 segments. A pair of spinal nerves comes out of each segment. The region of the spinal cord from
which a pair of spinal nerves originates is called the spinal segment. Both motor and sensory nerves are located
in the spinal cord.
The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40
grams. It lies within the vertebral column, the collection of bones (back bone).
Other parts of the central nervous system
The meninges are three layers or membranes that cover the brain and the spinal cord. The outermost layer is the
dura mater. The middle layer is the arachnoid, and the innermost layer is the pia mater. The meninges offer
protection to the brain and the spinal cord by acting as a barrier against bacteria and other microorganisms.
The Cerebrospinal Fluid (CSF) circulates around the brain and spinal cord. It protects and nourishes the brain
and spinal cord.
Neurons
The neuron is the basic unit in the nervous system. It is a specialized conductor cell that receives and transmits
electrochemical nerve impulses. A typical neuron has a cell body and long arms that conduct impulses from one
body part to another body part.
There are three different parts of the neuron:



the cell body
dendrites
axon
Cell body of a neuron
The cell body is like any other cell with a nucleus or control center.
Dendrites
The cell body has several highly branched, thick extensions that appear like cables and are called
dendrites. The exception is a sensory neuron that has a single, long dendrite instead of many dendrites. Motor
neurons have multiple thick dendrites. The dendrite's function is to carry a nerve impulse into the cell body.
Axon
An axon is a long, thin process that carries impulses away from the cell body to another neuron or tissue. There
is usually only one axon per neuron.
Myelin Sheath
The neuron is covered with the Myelin Sheath or Schwann Cells. These are white segmented covering around
axons and dendrites of many peripheral neurons. The covering is continuous along the axons or dendrites except
at the point of termination and at the nodes of Ranvier.
The neurilemma is the layer of Schwann cells with a nucleus. Its function is to allow damaged nerves to
regenerate. Nerves in the brain and spinal cord do not have a neurilemma and, therefore cannot recover when
damaged.
Types of neuron
Neurons in the body can be classified according to structure and function. According to structure neurons may
be multipolar neurons, bipolar neurons, and unipolar neurons:
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Multipolar neurons have one axon and several dendrites. These are common in the brain and spinal cord
Bipolar neurons have one axon and one dendrite. These are seen in the retina of the eye, the inner ear, and the
olfactory (smell) area.
Unipolar neurons have one process extending from the cell body. The one process divides with one part acting
as an axon and the other part functioning as dendrite. These are seen in the spinal cord.