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
The Human Body Background notes 7 7. The digestive system: the ins and out Our digestive system is a long tube that winds through the body from the mouth to anus. Food usually takes about a day and a half to pass from one end to the other. If the digestive tract of an adult was stretched out, it would measure almost nine metres in length. The digestive system breaks food down into particles that are small enough to be absorbed into our blood. Each part of this tube and the organs that are attached to it, has its own role in the digestion of food. The chemical energy that our bodies need comes from the food we eat. The mouth and oesophagus What might appear on our plate as a delicious meal is soon mashed beyond recognition by our teeth and tongue. By the age of three we have 20 milk teeth. By the time we reach 20 years of age we may have a total of 32 teeth. We have different shaped teeth to perform different tasks. We have incisors with sharp edges for cutting; canines with pointed tips for tearing; and molars with flattened ridged surfaces for grinding. Our teeth are covered by enamel, the hardest substance in the body. Proper oral hygiene is essential so that the enamel doesn’t start to break down. When we eat, our tongue positions food for chewing, mixes in saliva, and shapes each mouthful into a bolus. The tongue pushes this to the back of the mouth for swallowing. Saliva moistens food to help us chew and swallow. The salivary glands in our lower jaw release saliva when we see and smell food. Saliva is mostly water however it contains antibacterial substances, called lysozyme, that protect the mouth against infection. Saliva also contains the enzyme amylase that begins the process of digestion of carbohydrate foods such as cereals, breads, rice, fruits and vegetables. The chewed and moistened bolus is pushed down the oesophagus to the stomach by a wave of muscular contractions, called peristalsis. When we vomit, peristalsis works in the opposite direction to bring food from our stomach out of our mouth. A bolus of food moves down the oesophagus by peristalsis The stomach An empty stomach holds the contents of half a teacup but when full it can expand to hold half a bucketful. The stomach’s lining has many folds that stretch out as the stomach fills and collapse as it empties. Millions of tiny openings dot the lining of the stomach. These are the gastric pits, which are the source of gastric digestive secretions. Glands at the bottom of each pit produce gastric juice. Gastric juice is made up of watery mucous, hydrochloric acid and other digestive substances such as the enzyme called pepsinogen that helps to breakdown the protein foods in our diet. http://museumvictoria.com.au/education/ A Museum Victoria experience. 44 The Human Body Background notes 7 Food may stay in the stomach for several hours soaking in gastric juice and being broken down into smaller and smaller pieces. The stomach’s muscular walls churn its contents regularly to ensure thorough mixing. When the partly digested food resembles thick soup, the stomach lets it out through a little opening at one end. The partly digested mixture is called chyme. The stomach is able to absorb certain chemicals like aspirin and alcohol but it does not absorb other digested food particles. The small intestine Partly digested food oozes into the small intestine where digestive juices from our liver, gall bladder and pancreas are squirted and mixed into the food. The small intestine is where most digestion and absorption take place. Food usually spends about six hours being digested in the small intestine. The digested liquid slowly moves along by muscular contractions The lining of the small intestine has many folds. These folds increase the surface area of the small intestine three fold. Poking from these folds are millions of tiny finger-like projections, called villi, which further increase the surface area of the small intestine by another 10 fold. If the villi in a small intestine were flattened out, they would form a surface as large as a tennis court. Villi are full of tiny blood vessels. As food is broken down into smaller and smaller pieces in the small intestine, it moves across the walls of the villi into the blood. Because the surface of the small intestine is so large a lot of nutrients can be absorbed into the blood at one time. Goblet cells secrete mucous to protect the stomach from acid attack. Source: Monash University. Finger-like projections called villi in the small intestine. Source: University of Melbourne Two important organs: Liver and pancreas Blood carrying nutrients from our small intestine goes directly to our liver. Our liver is a very important organ. It processes and stores nutrients and sugar absorbed by our small intestines. It destroys poisons and bacteria and breaks down chemicals such as alcohol, so they are less harmful to the body. The liver recycles dead red blood cells and processes the red pigment into green bile. Soapy bile is stored in our gall bladder. It is released from a tube into our small intestine when we eat a fatty meal. Bile helps to break down fat into small globules that can be absorbed across the villi, into the bloodstream. The pancreas is another important organ that aids in the digestion of food. It produces the enzyme amylase to digests carbohydrates, proteases to digest protein and lipases to digest fats. It produces the hormones, insulin and glucagon, The liver processes and stores nutrients; destroys poisons and bacteria; breaks down chemicals; that help regulate the level of sugar in our blood and it also recycles dead red blood cells and makes bile. secretes an alkaline substance, sodium bicarbonate (Na2CO3) which neutralises the acid secretions from the stomach. http://museumvictoria.com.au/education/ A Museum Victoria experience. 45 The Human Body Background notes 7 The large intestine The large intestine is primarily a drying and storage organ. The wall of the large intestine also has many folds in it to increase its surface area. The large intestine absorbs water from the sloppy mixture of waste that comes in from the small intestine. To keep the waste contents moving along, the large intestine has many mucus-producing cells in its lining. Mucus lubricates the passage of wastes and protects the lining of the large intestine from acids and gases. The final segment of the large intestine is the anal canal, which finishes at the anus. A round muscle, called a sphincter, allows us to voluntarily open and close our anus when we need to. The left over solids that pass out of the digestive tract are called faeces and these contain undigested cellulose, bilirubin, bacteria, salt and water. Millions of bacteria live in our large intestine. Many of these bacteria keep our gut healthy and help process waste material. Unfortunately these bacteria also produce about half a litre of foul-smelling gas a day. Bacteria give faeces its distinctive brown colour. The green bile, which is secreted into the intestines from the liver, turns brown after being digested by bacteria in the bowel. Sometimes harmful bacteria infect our digestive tract. The worm-like appendix is where immune cell are produced to fight off the invaders. E.coli bacteria (above) & lactobacillus (below) are common bacteria found in our large intestines. Source: AMGEN Australia and Yakult Australia Pty Lty When things go wrong Gallstones Crystals can sometimes form in bile if cholesterol levels are high. These crystals are called gallstones. Gallstones can cause extreme pain and sometimes even damage to the gall ducts if they begin to move through it. Sometimes they have to be removed. What is heartburn? When food travels to the end of the oesophagus a valve, called a sphincter, opens to let it enter the stomach. Heartburn occurs when some of the acidic contents of the stomach refluxes back up through a weak valve and burns the oesophagus. Gall stones. Source: Royal Melbourne Hospital Gastric ulcers Bacterial infection is the cause of some stomach ulcers. Bacteria live in the stomach lining, destroying its protective layer. This infection may develop into a stomach ulcer. Diarrhoea We get diarrhoea when food waste rushes through the large intestine, leaving little time for water to be reabsorbed. This water is passed through the anus in very liquid faeces. Some bacteria can irritate the large intestine and affect its normal function, producing diarrhoea. Prolonged diarrhoea can cause dehydration. Constipation We get constipated when food waste travels too slowly through our large intestine. In this case, too much water is absorbed and the faeces become hard and dry. Constipation can be caused by not drinking enough water and by not eating enough high-fibre foods. http://museumvictoria.com.au/education/ A Museum Victoria experience. 46 The Human Body Background notes 7 In order to survive, many things must go into our bodies and many things must also come out. Our heart pumps blood to every part of our body, delivering a constant supply of nutrients, oxygen and hormones (chemical messengers that tell the body what to do). Our blood also picks up our bodies waste so that it is disposed of by our liver, kidneys, skin and lungs. An endless flow of blood carries all of these substances through our circulation. Our respiratory system: we need oxygen Our cells need oxygen, all the time. They produce carbon dioxide, a waste that must be removed. We obtain oxygen and expel carbon dioxide by breathing. When we breathe in, our diaphragm contracts (pulling our chest cavity downward) and air is drawn into our lungs. Our lungs contain millions of tiny air spaces, called alveoli. Alveoli walls are a single-cell thick and are covered in tiny blood vessels. This enables the gases to move across concentration gradients, between the alveoli and blood. Oxygen moves out of the lungs and attaches to red blood cells in the surrounding blood vessels. At the same time, carbon dioxide gas leaves these vessels and enters the alveoli spaces. When we breathe out, our diaphragm relaxes and this air is forced out of our lungs. The lungs, heart and excretory organs Sinuses: filtering air Our nose and sinuses moisten, warm and filter air as we breathe it in. Dust, bacteria and other particles are trapped by nose hairs and by mucus secreted by glands in our sinuses. Mucus is either blown out of the nose or is swallowed. Windpipe: channeling air Air passes down the trachea (windpipe) towards our lungs. The trachea branches to enter each lung. Tiny hairs called cilia sweep foreign particles trapped in mucus towards the throat, for swallowing. Smoking destroys cilia so coughing is the only way to remove mucus from the lungs. Alveoli in lung tissue Larynx: directing air Our larynx acts as a switch, directing food and air down the correct passageways. A spoon-shaped cartilage, the epiglottis, seals off the air passage when we swallow. If food enters the trachea, the epiglottis initiates a cough. Coughing pushes the food in a rush of air that can travel at 70 kph. Vocal cords: vibrating air Inside the larynx, vocal cords vibrate as air passes up between them. Like guitar strings, varying the length and tension of the vocal cords changes the pitch of the sound produced. The loudness of a voice depends on the force of the air-stream rushing across the vocal cords. Our nose and mouth connect to the back of the throat. http://museumvictoria.com.au/education/ A Museum Victoria experience. 47 The Human Body Background notes 8 8. The circulatory system: the round trip Circulation of blood Our blood supply never stops. Our cells demand nutrients and oxygen constantly. Our heart pumps blood around our body at 2.5 litres a minute. Blood surges away from the heart to the tissues of our bodies in arteries and returns in veins. A round trip from the heart to the toes and back takes a minute. The watery part of blood (plasma) carries dissolved nutrients and wastes. Floating in the plasma are billions of cells. Most of the cells are red blood cells, which carry oxygen. Some cells are white blood cells, which attack intruders like bacteria. The heart is two pumps joined together. The right side of the heart pumps blood, low in oxygen, to the lungs. The left side of the heart pumps blood, filled with oxygen from the lungs, out of the heart to the rest of the body. Blood flows into the heart through the vena cava and the pulmonary artery, via the left and right atria respectively. It then flows down into the left and right ventricles. As the ventricles contract they force blood out of the heart through the pulmonary vein and the aorta, to the lungs and the body, respectively. Blood flow through arterial circulation and the heart. Circulation through the heart, blood vessels and chambers. Blood vessels form a circuit Our bodies contain over 100,000 km of blood vessels. Arteries carry blood from the heart around the body. They get smaller and smaller as they branch out. The smallest arteries become tiny capillaries, only one cell wide. In the capillaries, nutrients and oxygen are exchanged for wastes. Capillaries eventually become veins, which carry blood back to the heart. Arteries: from the heart Under high pressure, blood travels away from the heart through muscular arteries that have thick elastic walls. This movement of the artery walls can be felt as a pulse. Most arteries, except those that take blood to the lungs, carry red, oxygenated blood. Cross section of connective tissue. Source: University of Melbourne . Veins: back to the heart Veins have thin walls. They carry blood back to the heart under relatively low pressure. Most veins, except those that take blood from the lungs, carry deoxygenated blood, which appears blue. Veins have valves to stop blood flowing backwards. Capillaries: so thin Capillaries are so narrow that red blood cells travel through them in single file. Capillary walls are only one cell thick. The walls of capillaries are so thin that nutrients, wastes and hormones can easily pass between the cells of the body and the blood. http://museumvictoria.com.au/education/ A Museum Victoria experience. 48 The Human Body Background notes 8 Our excretory system Cells dump their wastes into our bloodstream. Most of these wastes are removed from our blood by our kidneys. Kidneys: the filtering unit Each kidney has over a million tiny blood-filtering units called nephrons. Blood rushes into the nephrons at high pressure and is filtered through a cluster of tiny capillary membranes called a glomerulus. A glomerular capillary membrane is more than one hundred times more permeable than other capillaries in the body. Blood cells and plasma proteins are too large to fit through these tiny filtering units, however, plasma containing liquid wastes is filtered through into a collecting chamber called Bowman’s capsule. Nephron showing the Bowmans capsule the convoluted loop of Henle and the collecting ducts which drain into a ureters. This filtrate travels into a long tube called the loop of Henle, where most of the water and salt in the filtered plasma is absorbed back into our blood. The filtrate is then drained into a collecting duct which leads into a ureter and into the bladder. The liquid wastes and excess water becomes urine. Every day, our kidneys filter 1500 litres of blood to produce 1.5 litres of urine. Ureters: tubes for moving Urine flows from the collecting tube in the kidneys to the bladder through two tubes called ureters. Sometimes salts in urine can crystallise, forming a kidney stone. A large stone may block a ureter and prevent urine drainage. Pressure builds up in the kidney, causing severe pain. Kidney stones can be shattered by ultrasound. A magnified view of Bowman’s capsule, with glomerulus – showing magnified glomerular filtering membrane. Bladder: storage unit Our bladder is a collapsible muscular sac that stores urine until it is convenient to urinate. The urge to urinate is felt when the bladder contains about 200mL of urine. However, a bladder can hold up to a litre if necessary. Urethra: tubes for letting it out Urine leaves our bodies through the urethra. A man’s urethra is about 20 cm long and also carries semen. A woman's urethra is about 4 cm long. Bacterial infection can quickly spread from a woman’s urethra to the bladder and kidneys. Prompt medical attention is needed to prevent kidney damage. Urine: waste Urine consists of 96% water. Urea and creatine are cellular waste products produced from the breakdown of proteins. Their concentration in the urine varies with the amount of protein eaten in the diet and cellular activity. Urine also contains uric acid, which is produced from the breakdown of cellular nucleotides from old cells. The amount of ammonia in the urine depends on the amount of acid substances that are being metabolised by the kidneys. Salts such as sodium, chlorine, calcium, potassium, phosphorus and sulphates are also excreted by the kidneys and the urinary system http://museumvictoria.com.au/education/ A Museum Victoria experience. 49 The Human Body Background notes 8 When things go wrong: blocked passageways Asthma Asthma is often triggered by allergens such as dust mites or cat fur. Airways become inflamed and swollen, reducing airflow. The symptoms of this disease include shortness of breath and wheezing. Atherosclerosis High cholesterol levels in the blood may cause arteries to become blocked, a condition known as atherosclerosis. If the blockage occurs in the brain it can cause a stroke. If the blood supply to the heart is blocked it can cause a heart attack. People who smoke and eat fatty food are much more likely to suffer from atherosclerosis. Polycystic kidney disease This inherited disease affects 1 in 500 people. Cysts develop in the kidneys and can eventually cause them to fail. The symptoms are difficulty in breathing, swelling in the abdomen and diarrhoea. The cysts are not felt until a person's 40s and may not be a problem until his or her 60s. http://museumvictoria.com.au/education/ A Museum Victoria experience. 50 The Human Body Background notes 9 9. The muscular and skeletal systems: the power within We walk, talk, chew, smile, breathe and blink. We also lift, carry, stretch and strain. All of our movements depend on our musculoskeletal system, a bony skeleton with muscles attached. Our musculoskeletal system enables us to perform an enormous range of tasks. From threading a needle to chopping wood, our bones, joints and muscles work together for precision and strength. The skeletal system Our skeleton is the body's supporting framework. When muscles pull on the skeleton, movements are made. A skeleton has bones, joints and cartilage and makes up 20% of our body weight. Joints are the meeting point of adjoining bones that can move. Smooth cartilage covers the ends of bones at joints and enables them to slide over each other. Cartilage is also found in parts of the skeleton that need to bend and be flexible. Bones provide structure and strength; by weight, they are stronger than steel. To stay strong, bone tissue is constantly renewed. When bones break, they repair themselves. Soft tissues, like the brain and heart, are protected by bone. Bones store minerals for use by the body. Bone marrow produces new red and white blood cells. The Human Skeleton Bones The human skeleton can be divided into two main groups of bones. The first group includes the bones that protect and support our vital organs and tissues in the head and chest regions of the body, including the skull, spine and ribs. The second group supports our limbs and enable us to move. They include the bones of the shoulders, arms, pelvis and legs. Bones are shaped according to the role that they carry out in the body. Different shaped bones do different things. There are long bones like the femur (thigh), round bones like the patella (kneecap) and flat bones like those found in the skull. There are also irregular shaped bones such as vertebrae. Some bones are locked together and do not move, while others move at a joint. (Left to right) : Cross section of the shaft of long bone. Source: Monash University; Micrograph of compact bone that shows the dark osteons that house the bone cells (osteocytes).Source: University of Melbourne. http://museumvictoria.com.au/education/ A Museum Victoria experience. 51 The Human Body Background notes 9 What’s inside bone? A long bone is made up of an outer layer of compact bone with spongy bone at the ends. A cavity in the centre is filled with bone marrow. Hard compact bone is made up of tightly packed rods of bone. It is riddled with canals and passageways used by nerves, blood vessels and lymphatic vessels. Soft bone marrow is a jelly-like substance often found within spongy bone at the ends of long bones. Blood cells A magnified image of a single bone cell called an are replenished from stem cells that exist in the marrow. osteocyte. Source: National University Hospital of Singapore Osteocytes are cells that secrete the mineral rich bone. They are tiny spider-shaped cells found between the canals of compact bone. If they die, bone begins to break down. Hyaline cartilage cells produce a more flexible substance than bone cells do. Cartilage provides support as well as flexibility for the body. It covers the ends of long bones, supports the tip of the nose and ears and also provides the flexible support of the trachea. A magnified image of hyaline cartilage. Source: Monash University Our Joints – Fixed and Mobile Movement of the skeleton occurs at joints, places where bones meet. The structure of our joints enables us to move the way we do. Some joints allow bending movements; some allow twisting. In these mobile joints, the bones are protected by cartilage. The smooth cartilage and a slippery fluid lubricate a joint's movement. Gristly bands, called ligaments, are attached to the bones around a joint, holding them in position. Not all joints move. Some bones, like the skull plates, are locked rigidly in position. What do the joints do ? The hinge joint allows for bending of the elbow and knee. The ball-and-socket joint gives the shoulder and hips the greatest range of movement. The gliding joints in the hands and feet are restricted by ligaments and make very small movements. The saddle joint allows the thumb to rock back and forth and side to side. It allows pinching and gripping. The rotating joint is responsible for turning the head from side to side. hinge joint ball and socket http://museumvictoria.com.au/education/ gliding joint saddle joint rotating joint A Museum Victoria experience. 52 The Human Body Background notes 9 Muscles mean movement No part of our body moves without muscles. Muscles move the food in our intestines. They even make the irises of our eyes open or close to adjust to the light. Movements, which are beyond our conscious control, are made by smooth muscles. Cardiac muscle, found only in the heart, causes it to contract. Skeletal muscles, the muscles attached to bones, cause all voluntary movements to the body. They tend to work in pairs, relaxing and contracting in turn to move a joint. How do muscles contract and relax? When a muscle cell receives a signal, the filaments within it slide over each other and cause the muscle to contract. As the filaments release and slide apart again, the muscle relaxes. Skeletal muscle and smooth muscle cells are supplied with nerve endings that control their activity. However, most cardiac muscle cells contract automatically. A section of the heart, called the pacemaker, sends out electrical signals that pass from one muscle cell to another and cause the cells to contract at the same time. Bundles of muscles Muscles are made up of hundreds or thousands of very long, cylindrical cells joined together. Some skeletal muscle cells can be up to 30 cm long, but they are thinner than hair. Skeletal muscle: appear to have stripes under the microscope Smooth muscle does not have stripes. The cells are tapered and elongated. Skeletal muscle. Cardiac muscle is made up of branched cells that help electrical signals pass through quickly, causing the heart to contract rhythmically. When Things Go Wrong: Osteoporosis Osteoporosis is a disease that causes bones to become brittle. Women over 40 and men over 60 are at the highest risk of developing osteoporosis. Adequate calcium in the diet and plenty of exercise can reduce the chances of developing osteoporosis. Smooth muscle. Arthritis Osteoarthritis, the most common type of arthritis, is caused by overuse of a joint. It affects such weight-bearing joints as knees and hips. Cracks in the cartilage lining may cause it to become swollen Cardiac muscle and painful. Rheumatoid arthritis is a disease that leads to inflammation in the joints. They become very swollen and movement is severely hampered. http://museumvictoria.com.au/education/ A Museum Victoria experience. 53