Human body _ a visual encyclopedia

Each spot is a colony of thousands of bacteria that grew from a single bacterium in the dirty handprint. SURGERY DISEASE DEFENSES WOW! If disease or injury damages an organ inside the body, doctors may need to Given warmth and perform surgery to repair the damage. sufficient food, a single Patients are first given drugs called bacterium can divide and anesthetics to make them fall asleep and to prevent pain. The surgeon then give rise to 5 billion makes precise cuts in the skin in order trillion offspring in to open up the body and gain access to the damaged area. Surgeons work in just one day. spotlessly clean operating theaters to prevent germs from entering the patient’s body during an operation. FIGHTING CANCER Sometimes body cells stop working properly and begin to multiply out of control like germs, forming lumps known as tumors. This deadly disease is called cancer. Cancer is difficult to treat because drugs that attack cancer cells also harm healthy body cells. To help destroy tumors, doctors fire beams of powerful radiation at them—a treatment called radiotherapy. ▶ RADIOTHERAPY This patient is being treated for skin cancer. A beam of radiation is pointed at a tumor from several different angles, killing the cancer cells without harming the healthy tissue around it. 99

DISEASE DEFENSES Spare parts ▲ GLASS EYES Artificial eyes look realistic but they do not In the past, losing an important part of the restore vision. However, scientists body due to disease or injury led to death or are developing implants that severe disability. Today, doctors can repair many could restore some form of vision parts of the body with artificial replacements. to people who are blind. In years to come, they may be able to replace organs with fresh new ones grown in a lab from your own body cells. This technology would allow us to live decades longer than today. REBUILDING THE BODY Some of the artificial parts that doctors use to repair damaged bodies are mechanical or electrical devices made of metals and plastics. Others, such as artificial skin or transplanted organs, are made of living tissue. Most of these spare parts do not work as well as the organs they replace. ▶ NEW SKIN Scientists ▼ HIP JOINT Damaged by have discovered how to age or disease, some joints, such grow sheets of artificial as the hip, can be repaired skin from a person’s skin with an artificial replacement. cells. Although they’re very thin, these sheets may soon be used to repair damage caused by severe burns. STEM CELLS Some of the cells in the human body have an amazing ability: They can multiply and develop into any kind of tissue, such as skin, blood, or nerves. Such cells are known as stem cells. Scientists are trying to harness this ability of stem cells and create new tissues or organs to repair or replace body parts that are injured or diseased. 100

BLADE RUNNER DISEASE DEFENSES South African athlete Oscar Pistorius lost the lower part of both his legs when he was a baby. He now walks and runs on J-shaped artificial feet made of carbon fiber. These are so good that Pistorius, nicknamed “Blade Runner” by his fans, is able to compete against able-bodied athletes, as in the 2011 World Athletics Championships, shown below. ▲ EAR IMPLANT A cochlear implant picks up sound and transmits it to the inner ear, giving some sense of hearing to people who are deaf. PACEMAKER ▲ A computer ORGAN TRANSPLANTS Implanted into the chest controls the motors wall, a pacemaker sends an that make the arm The most common way of replacing electric signal along a wire and hand move. a damaged internal organ is to use a to the heart, controlling healthy organ from a donor—an the rate at which it beats. ◀ ROBOTIC ARM organ transplant. Transplants can This high-tech mechanical save lives and make people healthy arm is controlled by thought again, but recipients must take alone. Sensors on the shoulder powerful drugs to stop their bodies pick up nerve signals inside from rejecting the new organs. the wearer’s body and relay them to a computer, which controls the arm’s movements. A motor inside the elbow makes the arm bend. ◀ NEW BLADDER The bladder is one of the few organs that scientists can grow in a lab, using a patient’s own body cells. ▲ KIDNEY TRANSPLANT Surgeons prepare a donated kidney before implanting it into a recipient. 101

DISEASE DEFENSES Ancient remedies WOW! Before the age of science, people used all sorts of weird and People used to think wacky remedies to try and cure diseases. Many people visited diseases were spread by witch doctors or priests in search of magical cures. Others foul smells. Powerful made their own medicines, following traditional old wives’ perfumes were used to recipes. Many of the ancient remedies were useless or even dangerous, but a few really did work and are still used today. ward off illness. HERBAL MEDICINE Plants have been used as medicines all over the world for thousands of years and they are still popular today. Some of our modern drugs were discovered thanks to ancient herbal remedies. The painkiller aspirin, for instance, comes from the bark of willow trees, which people once used to make a pain-relieving tea. ▶ BROKEN ARM This engraving from the 1500s shows how a man’s broken arm was treated. Turning the screw pulled the arm straight, moving the broken bones back into position. 102

CUPPING A HOLE IN DISEASE DEFENSES THE HEAD The ancient practice of cupping was once widely Stone age people believed believed to help cure diseases that diseases were caused by encouraging blood flow. by evil spirits. The cure, Small cups are heated and they thought, was to drill placed on the skin. As the air a hole in the skull to inside a cup cools, it contracts, release the spirit— sucking up an area of skin and a technique called drawing blood to the surface. trepanning. Some Red rings and sometimes prehistoric skulls found by bruises are left afterward. archaeologists have as many as 40 holes drilled in them. BARBER SURGEONS BLOODLETTING In medieval Europe, all operations from tooth extraction to mending broken arms were performed Removing blood, or by barbers. Surgery was often deadly since barbers bloodletting, was a had little idea how to stop wounds from getting very common medical infected. And patients had to stay practice for centuries. It awake during treatment since was thought to put body general anesthetics fluids back into balance. didn’t exist. Blood was drawn by cutting the skin, by ▲ BLOODSUCKERS piercing veins and The Roman Emperor Galerius arteries, or by using was treated with leeches to bloodsucking leeches. cure an “abominable stench.” MUD BATHS Bathing in pools of mud, hot springs, or mineral water has long been a popular treatment for joint pain (arthritis), skin complaints, and other conditions. Even today, people travel great distances to visit famous spa towns and “take the waters.” 103

LUNGS AND BREATHING LUNGS AND BREATHING

The cells in your body can’t LUNGS AND BREATHING survive without a continual supply of oxygen from the air. Your lungs take in air with every breath, bringing oxygen to your blood to be carried around the body.

Airways Air is warmed and cleaned as it We never really think about breathing because it passes through the nasal cavity. is such an automatic process, but it is vital to our LUNGS AND BREATHING survival. Every inward breath brings a new supply The epiglottis of air into the network of tubes that make up the is a flap at the respiratory system. These take air deep into the lungs, front of the voice box. where the gas oxygen, which is vital for life, passes into the blood to be carried all around the body. The voice box, or larynx, produces sounds. THE RESPIRATORY SYSTEM Your lungs and the airways that carry air in and out of your body are located in your head and chest. The airways include the nasal cavity, the throat, the larynx (voice box), and the branching passages inside your lungs. Incoming air is cleaned as it travels through your nose and trachea to remove dirt and germs that could block or damage the delicate lungs. Oxygen ▶ THIS WAY TO THE LUNGS The ridged appearance of the trachea is produced by rings of rubbery cartilage that hold it open during breathing and stop it from closing up. OXYGEN SUPPLY We breathe in oxygen and breathe out waste carbon dioxide. So why doesn’t our oxygen supply run out? Plants ensure there is a steady supply in the air by releasing oxygen during photosynthesis, the process that uses sunlight and carbon dioxide to make food. Carbon dioxide 106

NO CHOKING Every time you swallow food or drink, your breathing stops. A flap of tissue called the epiglottis drops over the entrance to your voice box to stop food from getting into the airway to your lungs and making you choke. If this happens, however, coughing forces food out of the airway and back into your throat. Breathing LUNGS AND BREATHING ▲ AIR CLEANERS The epiglottis Inhaled air The trachea is lined with sticky is upright. Esophagus mucus that traps dirt and germs Air flows in the incoming air. Tiny cilia toward the lungs. (green) move the dirty mucus to the throat, where it is swallowed. Swallowing Air is carried to Food is pushed and from the lungs into the by the trachea. esophagus. Curved ribs form The epiglottis folds a protective cage over to cover the around the lungs. entrance to the voice box. The airways become more finely branched as they penetrate deeper into the lung. The heart pumps WOW! blood to the lungs to pick up oxygen. Every day, you swallow about a Protective glassful of slimy membranes mucus produced by cover the lungs. your airways. A sheet of muscle called the diaphragm helps inflate the lungs. 107

In and out AIR IN Your body cells need constant supplies of oxygen. Air is drawn into the body by the They also need to get rid of waste carbon dioxide movement of the rib muscles and gas. This exchange of gases happens in your lungs. the diaphragm—a sheet of muscles Breathing in and out ensures that fresh under the lungs. When you breathe supplies of oxygen are brought in, the diaphragm tightens and pulls in and stale air containing down. At the same time, your rib carbon dioxide is removed. muscles tighten and pull your The lungs cannot move by ribcage upward and outward. themselves, so the process relies on muscles in the chest As a result, your chest gets to suck air in and push it out. larger and your lungs suck LUNGS AND BREATHING in air through your mouth and nose. You take in about 40,000 breaths of air every day. Oxygen (20.8%) Nitrogen Nitrogen and water vapor (79.16%) Carbon dioxide (0.04%) Oxygen The ribcage moves upward and Other outward as the rib gases muscles contract. WHAT IS AIR? The lungs get bigger Air is a mixture of gases and suck that surrounds our planet. in air. As you can see from this balloon diagram, air is The mostly nitrogen (about diaphragm 78 percent) and oxygen flattens (about 21 percent), plus and pulls small amounts of other downward. gases. Oxygen is the only part of the gas mixture that your body uses. 108

AIR OUT CONTROL CENTER LUNGS AND BREATHING When you breathe out, your diaphragm relaxes You don’t have to think about breathing. The brain stem and springs back into its natural, curved shape, pushing (orange) at the base of your brain (green) automatically up against your lungs. At the same time, your rib muscles controls your breathing rate. This control center monitors the relax and let your ribcage drop back downward and amount of carbon dioxide in your blood. If the carbon dioxide inward. As a result, your lungs are squeezed and air is level rises, during exercise for example, your breathing rate pushed out through your mouth and nose. The air you goes up to supply more oxygen to your muscles and to flush breathe out contains less oxygen and more carbon out the excess carbon dioxide. dioxide than the air you breathed in. The brain stem controls the body’s breathing rate. 80.4% Nitrogen and water vapor 15.6% Oxygen 4% Carbon dioxide The ribcage moves downward and inward as the rib muscles relax. The lungs FAST OR SLOW? are squeezed, pushing out air. At rest, we breathe in and out between 12 and 15 times a This is minute. During exercise, such the size as running, our breathing rate of the chest can more than double and we when air is breathe more deeply. This is breathed in. because our muscles are working harder and need more oxygen to The release the energy required for diaphragm movement. They also produce relaxes and more carbon dioxide, which pushes must be removed. upward. 109

LUNGS AND BREATHING

LUNGS AND BREATHING BREATHING UNDERWATER A continual supply of oxygen from the air is vital for the human body. Without it, brain cells start dying in large numbers after only five minutes. Scuba divers breathe air underwater from a compressed-air tank carried on the back. One tank has enough air to last 45 minutes.

Inside the lungs The trachea carries air to and Packed into your chest are a pair of soft, spongy lungs. from the lungs. Their job is to get oxygen into, and carbon dioxide LUNGS AND BREATHING out of, your blood. This transfer is made possible by millions of tiny air bags inside the lungs that are supplied by a rich network of blood vessels. Blood is carried into the lung by the pulmonary artery. Blood is carried out of the lung by the pulmonary vein. LUNG STRUCTURE Air is carried to the lungs by a tube called the trachea. This branches into two tubes called bronchi, which branch further into smaller and smaller tubes called bronchioles. At the end of the tiniest bronchioles are 150 million little air bags, called alveoli, which swap oxygen for carbon dioxide. Blood vessel Air bag The right bronchus (alveolus) carries air to and from the right lung. ▲ AIR BAGS The bronchioles end in tiny, lumpy air bags called alveoli. Oxygen from Bronchioles the air inside alveoli passes into the blood capillaries that surround them. 112

The left bronchus BRONCHIAL TREE carries air to and from the left lung. The branching network of airways inside the lungs is sometimes called the bronchial LUNGS AND BREATHING tree. As you can see here, it looks like an upside-down tree with the trachea as the trunk, bronchi as the branches, and the tiny bronchioles as twigs. The left lung opened up to show its structure. LOOK CLOSER: DUST GOBBLERS When you breathe in air, most dust particles, pollen grains, and germs are trapped by sticky mucus in the nose and airways. But some of them travel deep into the lungs. Before they can do any harm, they are gobbled up by wandering macrophages. ▲ GERM EATER A macrophage is a type of white blood cell. It moves by extending its narrow “arms,” which it also uses to grab foreign particles. INTO THE BLOOD Oxygen passes into the blood. This is a close-up view of what happens at the junction between Blood cell an alveolus and a capillary. The heart Oxygen travels from the fits into alveolus into the this space. blood, and carbon dioxide moves in the opposite direction. Slippery membranes Inside of alveolus surround the lungs. Carbon dioxide passes out of the blood into the alveolus. 113

Coughs and sneezes Your lungs and airways are always at work, taking in fresh air and pushing out stale air, and most of the time you don’t LUNGS AND BREATHING notice them doing it. Occasionally, however, they might WOW! surprise you with an unexpected cough, sneeze, yawn, or hiccup. These processes mostly play a vital role in keeping you healthy, but some are a bit of a mystery. You can’t cough, sneeze, yawn, hiccup, or laugh when you’re dreaming, since most of your SNEEZING muscles are paralyzed during dreams. If something itchy gets in your nose, your body clears it out with a deafening blast of air: a sneeze. After a sharp intake of breath, your eyes close and your rib muscles tighten to squeeze your lungs and blast air out, often with a brief scream. Your tongue rises to direct air into your nose, but most of the blast often comes out of the mouth. germs out Sneezes clear o f your nose ut spread them to other people.b

HICCUPS Hiccups happen when your diaphragm muscle twitches. The sudden movement of the diaphragm causes a very sharp intake of breath, and the rush of air makes the vocal cords at the top of the trachea snap shut, blocking the air and producing a “hic” sound. Hiccups have no known purpose and can start for no apparent reason. They usually stop after a minute or two, but in rare cases they last longer. One man had a bout of hiccups that lasted 68 years. “Hic” “Up” LUNGS AND BREATHING Air rushes in. Air leaves. YAWNING The vocal The vocal cords No one knows what purpose yawning cords snap shut, open again. serves. Yawns aren’t an efficient way of blocking the air. breathing, and although we yawn when The diaphragm we’re tired, we also do it when we’re The diaphragm relaxes and nervous. To add to the mystery, yawns twitches and pulls moves up, are contagious—if you see someone flat, pulling air pushing air out. yawn, you’ll probably do it too. into the lungs. LAUGHING SNORING COUGHING When we laugh, the vocal cords at the The roof of your mouth is hard at the Coughing gets rid of specks of dirt top of the trachea open and close front but soft at the back, where it or germs that have gotten into your repeatedly, chopping our breaths into forms a flap of tissue separating your airways. First you take a deep breath. segments and making a “ha, ha, ha” or mouth and nose. In some people, this Then your lungs squeeze, but the “ho, ho, ho” sound. Laughter triggers flap wobbles around during sleep, vocal cords stay shut to build up the release of brain chemicals called making a loud noise—snoring. People pressure. When they open, air shoots endorphins, which make us feel great. snore less if they lie on one side. out in a sudden blast. 115

LUNGS AND BREATHING Making sounds Many animals make sounds, but humans are the only animals that can communicate using spoken words. Both speech and language are controlled by your brain. It sends out instructions that make your respiratory (breathing) system produce sounds that can be understood by those around you. WHAT IS SOUND? WOW! Throw a pebble into a pond and circular Men have deeper ripples will move outward from the point voices than women of impact. In the same way, if you pluck because their vocal cords are longer and vibrate a guitar string so it vibrates, sound waves will move outward through more slowly. the air. These sound waves are picked up by your ears. VOICE BOX The vocal cords in your voice box (larynx) make the sounds that are shaped into speech. These cords are separated when you are silent and breathing normally. When you speak, your brain sends signals to the voice box muscles to pull the vocal cords together and make them tight. When air pushes between these muscles, they vibrate and create sounds. ▲ VOCAL CORDS Sounds are The vocal ▲ VOCAL CORDS TAUT Bursts of air produced by cords stretch OPEN During normal passing between the the voice box across the breathing, the open taut vocal cords make (larynx). larynx. vocal cords allow air them vibrate and to move to and from produce sounds. The trachea the lungs. carries air to and from The esophagus the lungs. is a part of the digestive system. 116

SHAPING SPEECH FAST FACTS Under orders from your brain, the ■ Around 6,900 languages are spoken in muscles controlling your tongue, lips, the world today. The top five most spoken and cheeks shape the sounds coming languages are Mandarin Chinese, from your vocal cords. They create English, Spanish, Hindi, and Russian. a sequence of words, spoken in a ■ The vocal cords of a soprano—a recognizable language, which reveals woman with a high singing voice— your thoughts and feelings to others. vibrate at up to 2,000 times a second. ■ The vocal cords of a bass—a man Tongue muscles with a low singing voice—vibrate LUNGS AND BREATHING help shape sounds. just 60 times a second. IN CONTROL These two scans show which areas of your brain are used when you speak (left) and when you listen to spoken words (right). In the first scan, Wernicke’s area finds the correct words to match what the brain has to say and sends signals to Broca’s area. This, in turn, sends signals to the voice box and the mouth instructing them to produce speech. Broca’s area Speaking Wernicke’s area Listening The hearing controls speech. “understands” words. area of the brain MAKING MUSIC ▶ HEARING Sound Some people can use their breathing systems to make sounds waves from the speaker’s much louder than their own voices. Controlled bursts of air from mouth enter the listener’s this musician’s lungs create buzzing vibrations in her lips. These ears. Here they are detected sound vibrations enter and travel through the long tube-shaped by sound receptors that trumpet, which makes the sounds much louder. send signals to the brain. 117

FUELING THE BODY FUELING THE BODY

Just like a car, your body FUELING THE BODY needs a supply of fuel to keep you going. All your energy comes from food, which your digestive organs break down into simple molecules for your body to absorb.

Food for life FUELING THE BODY You need to eat food so that your body can get the nutrients it needs to Peppers supply minerals grow, and to repair and maintain itself. Some nutrients are needed in and vitamins and are a large amounts. These include carbohydrates, which supply energy, and good source of fiber. This proteins, which are used to build and repair your body’s cells. Vitamins helps speed up digestion. and minerals are needed in smaller quantities. WOW! A HEALTHY DIET Water is an important The nutrients you need to stay healthy part of your diet. Some can be found in food. Different types of food provide different nutrients. fruit contains a lot This platter shows how much of liquid—apples are you should eat of each type, compared to the others. 84 percent water. For example, you should eat a lot of fresh fruits and vegetables, but not very much meat or fat. Carrots contain vitamin A. You need this vitamin to keep the eyes healthy. BREAKING DOWN FOOD The nutrients that we need are often “locked” inside food and can’t be used by your body right away. Your digestive system breaks down complicated foods into simple sugars, fatty acids, and amino acids—the basic building blocks of protein—that the body can absorb and use. Fatty acid ◀ FATS These are broken down into simpler glycerol and fatty acid molecules. Fat Glycerol ◀ PROTEINS These are broken down into separate amino acids. Amino acids Protein Sugars ◀ CARBOHYDRATES Whole grains are a Complex carbohydrates, such good source of some Carbohydrate as starch, are converted into B vitamins. These help 120 simple sugars, such as glucose. keep your cells healthy.

▶ RICKETS This X-ray shows the VITAMINS AND MINERALS bent leg bones of a child with rickets. Rickets occurs when the body doesn’t Although they are only needed in get enough vitamin D. This vitamin tiny amounts, vitamins and minerals are essential for good health. A healthy is essential for normal bone growth. diet should include vitamins A, It is found in fish and eggs. Your B group, C, D, and E. Important body can also produce it when minerals include calcium, for healthy you get sunlight on your skin. teeth and bones, and iron, which is needed to make red blood cells. Citrus fruits, such as lemons, are a good source FUELING THE BODY of vitamin C, which can help you fight infections. Beans and nuts are a good source of protein, essential for cell growth and repair. Fish, meat, and LIND’S LIMES eggs are high in protein. Sweets and Sailors used to develop a deadly disease fats should be called scurvy on long sea voyages. In 1747, eaten in small a navy doctor named James Lind found amounts. that feeding them citrus fruits, such as limes, cured them. Later, scurvy was shown to result from a lack of vitamin C. FEELING HUNGRY The area of your brain that controls Hunger and fullness are your appetite. sensations produced by a part of your brain called the hypothalamus. Before a meal, the stomach releases a hormone that makes you feel hungry. After a meal, fat tissue releases another hormone that makes you feel full. Cheese and other dairy products are a good source of calcium, needed for healthy bones. Bread and other starchy foods are the body’s main source of energy.

Digestion Food is moved between the teeth and toward the FUELING THE BODY Food provides the body with nutrients. The teeth back of the mouth for But releasing those nutrients from food cut and swallowing by the tongue. requires a process called digestion. First, crush food into the food has to be cut up and churned small pieces. The throat (pharynx) into small particles. This makes it connects the mouth easier for enzymes to work on the Salivary glands to the esophagus. complex chemicals that make release saliva into up food and split them into the mouth. The epiglottis stops substances that can be used by food from going down the body. All of this happens The esophagus the trachea during inside the digestive system. transports food swallowing. from the throat to the stomach. The trachea, part of the respiratory system, carries air to the lungs. The stomach stores and partially digests food. DIGESTIVE SYSTEM Nutrients are processed by The digestive system consists of the liver. a long, muscular tube called the digestive tract, which runs from Fats are turned into tiny the mouth to the anus. It has droplets by bile, a green several distinct regions, including fluid released by the the esophagus, stomach, small gall bladder. intestine, and large intestine— each with its own tasks. Other The pancreas releases organs that help digestion, digestive enzymes into such as the gall bladder and the small intestine. pancreas, add digestive chemicals at various points Most food is digested in along the way. the small intestine. The nutrients that are released WOW! are absorbed into the bloodstream. During an average lifetime, a person will In the large intestine, eat their way through, water is absorbed from waste to form and digest, at least feces (poo). 20 tons of food. The rectum holds and expels feces. The anus is the opening at the end of the digestive tract. 122

ENZYMES IN ACTION Enzymes are proteins that speed Long food Long food up the breakdown of complex molecule molecule molecules into much simpler splits nutrients that can pass into the bloodstream to be used by cells. Short, Without enzymes, digestion simple nutrient molecules would be so slow that we would Enzyme Active site Enzyme ready of enzyme for action starve. Each enzyme acts on a FUELING THE BODY specific food type, such as the ▲ ATTACHMENT Inside the ▲ SPLITTING MOLECULES ▲ NONSTOP ACTIVITY enzyme pepsin in stomach juice digestive tract, a food molecule The active site breaks some of The enzyme releases the molecule that breaks down proteins into attaches itself to a region of its the chemical bonds in the food once it has split apart. One molecule, splitting it into smaller enzyme can process hundreds simpler nutrients. You can see specific enzyme called the nutrient molecules. of molecules a second. here how an enzyme works. active site. TAKING TIME 00:00:10 LOOK CLOSER: FOOD POISONING It only takes a few seconds for food chewed Ten seconds Most bacteria and other microorganisms in food in the mouth to arrive in the stomach. But, after swallowing, are destroyed by the harsh, acidic conditions as you can see from this stretched-out view food arrives in found in the stomach. But some harmful bacteria of the digestive tract, the rest of the the stomach. survive and they, or their poisons (toxins), irritate process takes up to two days longer. the stomach and intestines, giving you food This ensures that food is properly 03:00:00 poisoning. This causes symptoms such as digested to release as many nutrients vomiting (throwing up) and diarrhea (runny poo). as possible, and that Food in creamy, these nutrients are liquid form is all absorbed into squirted into the the bloodstream. small intestine. 06:00:00 Food is now almost completely digested and its nutrients are being absorbed. 20:00:00 08:00:00 ▲ STOMACH BUG This micrograph shows Salmonella, a bacterium that causes food Water is absorbed Watery, indigestible poisoning. The bacterium is rod-shaped and as waste passes along waste leaves the can move by wiggling its hairlike flagella to the large intestine. small intestine. push itself forward. Semi-solid feces 32:00:00 123 form as water is removed. Feces arrive in the rectum 20 to 44 hours after eating.

What a mouthful! FUELING THE BODY If you tried to swallow a large piece of food The parotid gland without chewing it, you would probably releases saliva choke. You must first grind it up by into the mouth. repeatedly bringing your teeth together to chop and crush it into smaller pieces. Esophagus While you chew, the food is moistened Trachea with saliva, and moved and mixed up by your tongue. In addition to making food moist, saliva contains an enzyme that starts to digest it. DOWN THE HATCH The roof of the mouth is called The first part of the digestive the hard palate. system, the mouth, contains the teeth and tongue. Slippery saliva A duct from the rushes into the mouth from three parotid gland carries pairs of salivary glands situated saliva to the mouth. under the tongue and in front of the ears. Saliva lubricates the The incisor teeth food as it is chewed into a pulp. cut up food as it A ball of food is then pushed enters the mouth. into the throat and travels down the esophagus to the stomach. The tongue moves Two pairs of glands food between the release saliva under teeth, and tastes it. the tongue. Canine Molars OPEN WIDE Premolars Adults have a full set of 32 teeth, which vary in shape and size according to what they do. Chisel-like incisors cut and slice food, while pointed canines grip and pierce. Farther back are the premolars and molars, which chew and grind food between the pointed tips of their crowns. Incisors 124

WOW! INSIDE A TOOTH ▲ ENAMEL is the hardest FUELING THE BODY material in the body and Your six salivary A tooth has two layers: a smooth enamel crown forms the crown of the tooth. glands release nearly that provides a hard biting surface, and the It consists of microscopic rods two pints (one liter) dentine beneath, which also forms the root. of calcium phosphate. of watery, slimy saliva Inside the dentine is the pulp cavity containing blood vessels that supply tooth cells, and The root of the tooth into your mouth nerve endings that detect pressure, heat, is embedded in a every day. and cold, and also signal pain. socket in the jawbone. Crown A band of tough tissue called a ▼ DENTINE is harder The gum forms a ligament anchors the than bone and forms the collar around the tooth in its socket. framework of the tooth. base of the crown. Its honeycomb structure Jawbone helps it resist the enormous Pulp is the soft squashing forces that are tissue containing Blood vessels created during biting. nerve endings supply the tooth and blood vessels. with nutrients. A nerve provides the tooth with feeling. FROM MOUTH TO STOMACH Swallowing, the process that moves food from the to push food into your throat. The other two stages, mouth to the stomach, has three phases. The first, in in the throat and esophagus, are reflex actions that the mouth, is under your control, so you decide when happen automatically. Soft Contracted palate muscle Food Food bolus bolus ▲ MOUTH As chewing Relaxed Movement reaches completion, the muscular muscle towards tongue molds food particles into Epiglottis stomach a bolus (ball) that is pushed into the throat. Esophagus ▲ THROAT Muscle ▲ ESOPHAGUS Rippling contractions push the bolus waves of muscle contractions towards the esophagus, while the in the esophagus wall push soft palate and epiglottis stop it the bolus to the stomach. from entering the trachea. This takes about ten seconds. 125

FUELING THE BODY In the stomach Within this area is the cardiac sphincter. It stops The stomach is a muscular “bag” that sits just below the food from flowing back diaphragm muscle in the middle of the chest. It has two up into the esophagus. key roles. When you eat, it releases an acidic gastric juice that starts to break down the proteins in ◀ PYLORIC food. It also stores food, releasing it at a SPHINCTER steady rate so that the small intestine A sphincter is a ring of has time to digest it efficiently. strong muscle. The pyloric sphincter, seen here through FILLING AND EMPTYING an internal camera, remains tightly closed as the stomach The process of filling and emptying the stomach depends on the processes food. type of food eaten, but it takes at least three hours. During this time, food is partially digested and made ready for the main part Folds in the of digestion in the small intestine. Only when the meal has been muscular walls turned into liquid form is it released into the duodenum. disappear as the stomach expands. Food is mixed ◀ DURING A MEAL with gastric As the stomach fills juice. and expands, recently chewed food is mixed with gastric juice by contractions of the stomach’s wall. ◀ 12 HOURS The wall of the AFTER EATING stomach contracts. Food is partly digested by gastric juice and churned by powerful muscular contractions into a creamy liquid The pyloric called chyme. sphincter keeps food in the SQUEEZE AND CHURN stomach. The stomach wall has three layers The pyloric ◀ 34 HOURS of muscles, arranged at different sphincter AFTER EATING The angles to each other. These produce opens. pyloric sphincter opens powerful contractions that squeeze the food and mix it with gastric slightly and the stomach juice. An enzyme called pepsin gets Chyme to work on food proteins, breaking (liquid food) wall contracts to push them down into simpler substances. small quantities of chyme into the duodenum, Chyme is squirted which is the first part into the duodenum. of the small intestine. 126

Your stomach can stretch to more t FUELING THE BODY han 20 times its size after eating. A JUICY TALE In 1822, fur trapper Alexis St. Martin accidentally shot himself in his side. This left a permanent opening to his stomach called a fistula. American army surgeon William Beaumont, who treated St. Martin, realized he could use his patient to find out how digestion in the stomach worked. Beaumont placed different types of food through the opening to see how fast they were digested. He also, as shown here, extracted gastric juice to investigate its composition and action on food. The longitudinal WHAT MAKES YOU SICK? muscle layer extends the Vomiting, or throwing up, happens when length of the something irritates the lining of the stomach. stomach. This triggers the vomiting reflex. The diaphragm and abdominal muscles contract, The circular muscle squeezing the stomach so that food is forced layer wraps around up the esophagus and out of the mouth. the stomach. Vomiting center in The oblique muscle the brain triggers layer runs diagonally vomit reflex around stomach. Soft palate blocks entrance to nasal cavity Epiglottis blocks entrance to trachea ▶ STOMACH Gastric pit Diaphragm LINING pushed This magnified view downward of the stomach’s lining shows the openings of the Pyloric glands that release digestive sphincter gastric juice into the stomach. closes Abdominal muscles push inward 127

FUELING THE BODY Gut The pancreas Liver produces enzymes and hormones. reactions The gall bladder At nearly 23 ft (7 m) in length, the stores and small intestine is the longest and most releases bile. important part of the digestive system. With the help of the pancreas and gall The large intestine lies in bladder, the small intestine completes front of the duodenum, the digestive process. It then absorbs the first part of the all the released nutrients so that they small intestine. can be used by your body’s cells. LONG AND WINDING JOURNEY The second part of the small This long, middle part of the digestive tract intestine is is coiled up to fit into the abdomen. The called the small intestine has three sections. The short jejunum. duodenum receives chyme from the stomach and digestive juices from the gall bladder and pancreas. The longer jejunum and ileum are where most digestion and absorption of food takes place. GALL BLADDER AND PANCREAS The gall bladder and pancreas Bile duct Appendix release fluids through a common duct into the duodenum. This Pancreatic The ileum is the kick-starts digestion in the small Pancreas duct final and longest intestine. The baglike gall bladder part of the small stores bile made by the liver. Bile intestine. turns fats into tiny droplets that are much easier to digest. The Gall bladder pancreas makes pancreatic juice. This contains enzymes that break down carbohydrates, proteins, and fats in the food we eat. Opening of Duodenum pancreatic and bile ducts 128

Muscular wall ◀ INSIDE VIEW This cross-section FUELING THE BODY of the small intestine reveals its structure. Muscles in its wall create wavelike contractions that both mix food and move it onward. The lining is covered with tiny, fingerlike bumps called villi. WOW! ▶ VILLI These are some of the millions The small intestine of microscopic villi lining is coiled inside the the small intestine. Villi provide a huge abdomen, but stretched surface area for digestion and absorption out it would be as long to take place. If stretched out, the villi in as four adults laid your small intestine would cover an area roughly the size of a tennis court. head to toe. Villi Capillary network Food molecule inside a villus in a capillary DIGEST AND ABSORB Inside the small intestine, enzymes attached to the surface of villi complete the process of digestion. They break down food into simple nutrients—glucose, amino acids, and fatty acids. ▲ IN THE MIDDLE The ▶ ABSORPTION Glucose and small intestine, which winds amino acids are absorbed into the its way from the stomach to the capillary networks inside villi and large intestine, takes up much are carried in the blood to the liver of the space in the abdomen. for processing. Fatty acids enter lymph capillaries (green) for their journey to the liver. Direction of blood flow 129

FUELING THE BODY BUMPY SURFACE The inside of your small intestine is lined with thousands of tiny bumps called villi. This magnified view shows the fingerlike extensions on the surface of a villus. These provide a massive surface area to speed up digestion and absorb nutrients.

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End of the line FUELING THE BODY The final section of the digestive tract, the large intestine, is twice the width of the small intestine but only one quarter of its length. The large intestine receives watery, indigestible waste from your small intestine. Here, it absorbs valuable water from that waste, forming semisolid feces (poo) that are pushed out of your body. THE BOWELS OF THE BODY The ascending One of three The transverse colon rises up bands of muscle colon passes across The large intestine has three parts—the the right side of that run along the abdomen, just cecum, the colon, and the rectum. The the abdomen. the length of below the stomach. longest section, the colon, forms and moves the colon. feces, which contain waste food, dead cells, and bacteria. They eventually arrive in the rectum and are expelled when you feel an urge to go to the bathroom. ▶ COLON This Circular cross-section shows the muscle layer muscles that produce the movements of the colon. A valve stops waste Feces inside The lining releases from flowing back into the S-shaped slippery mucus the small intestine. sigmoid colon. to moisten feces and make their passage easier. Band of The cecum is the The appendix lengthwise first part of the sticks out from muscle large intestine. the cecum. Lining of colon LOOK CLOSER: FRIENDLY BACTERIA Rectum Anus Your colon contains trillions of bacteria, The digestive process many of which are friendly or helpful to also produces waste the body. They digest material that human gases, which are enzymes cannot, releasing useful nutrients, expelled through the such as vitamin K, which the body absorbs. anus. Flatulence, or “passing gas” as it ▶ COLON BACTERIA is known politely, can The lining of the colon be noisy and smelly! (brown) is covered by bacteria (purple) that feed on undigested food. 132

MOVING THINGS ALONG As indigestible waste food is mixed and formed into that run the length of the colon. Movement of material feces, it is propelled along the colon by three types of along the colon is slower than in other parts of the movements (below). All are produced by contractions digestive system. This gives more time for as much of the circular muscles and the three bands of muscle water as possible to be absorbed from the waste. ▲ SEGMENTATION Every ▲ PERISTALSIS Alternating ▲ MASS MOVEMENTS FUELING THE BODY 30 minutes, the muscles contract waves of muscular contraction Three times a day, powerful to pinch waste into pellets that and relaxation push the pellets contractions force feces from the are mixed but not moved. towards the rectum. descending colon into the rectum. The descending colon WOW! WHY THE APPENDIX? travels down the left side of the abdomen. Always wash your The thin, dead-end tube called the appendix was once thought to have Pouches in the hands after going to the no function at all in humans. Scientists colon are called now believe that it plays a part in our haustra. bathroom. Feces are 50 body’s defenses, forming part of the immune system. It also stores friendly percent bacteria, and bacteria to replace those in the colon, in case they’re flushed out during an some are harmful. attack of diarrhea (runny feces). Bladder Feces are pushed downward by contractions of the rectum wall Anal canal Anus ◀ WASTE DISPOSAL ▲ XRAY OF THE APPENDIX links rectum Contractions of the colon push This colored X-ray image shows the to anus feces into the normally empty slender, fingerlike appendix located rectum. This stretches the rectum near to where the small intestine wall, triggering an urge to go (right) joins the large intestine (left). to the bathroom. Two powerful sphincter muscles, which keep the anus closed, relax. The rectum wall contracts and pushes the feces out through the open anus. Anal sphincter muscles relax to allow feces to pass through 133

Chemical A branch of the hepatic The inferior vena cava vein carries blood processed takes oxygen-poor blood by liver cells to the inferior back to the heart. vena cava (a major vein). factory The hepatic portal vein carries nutrient-rich blood from the stomach and intestines. FUELING THE BODY The largest internal organ, the liver, is your body’s chemical factory. It performs almost 500 different functions, including processing the nutrients released from food. It also makes the bile your digestive system uses to help break down fats. VITAL ORGAN The liver is important because it processes, stores, and makes so many substances. For example, it stores and releases energy-rich glucose, processes fats and amino acids, stores vitamins and minerals, breaks down toxins and drugs, and recycles red blood cells. It also releases heat, helping to keep you warm. Central vein removes Surface The bile duct processed blood of lobule collects bile from the liver cells. LIVER LOBULES Lobule shown in The liver contains around cross-section one million sesame seed–sized processing plants called lobules. Inside a lobule, liver cells are arranged in plates that radiate out like spokes from a central vein. The cells filter substances out of the blood as it flows towards the vein and add other substances to it. Branch of ▲ LIVER CELLS Also called bile duct hepatocytes, these cells perform hundreds of tasks, including ▶ LOBULE STRUCTURE Three Branch of Branch of hepatic storing glucose, making proteins, vessels run along each corner of the hepatic artery portal vein carries and removing toxins (harmful lobules. Two supply blood for supplies blood blood rich in nutrients substances) from the blood. processing. The third, a bile duct, for processing removes bile made by the liver cells. 134

The esophagus The hepatic artery LOOK CLOSER: BLOOD CLEANERS carries food from carries oxygen-rich the throat to the blood into the liver. The small blood vessels that pass between liver cells stomach. are lined with macrophages (cells that eat other cells) called Kupffer cells. They “clean” blood by removing worn-out red blood cells, along with bacteria and debris. The iron retrieved from red blood cells is recycled. Left lobe of liver FUELING THE BODY The stomach lies ▲ KUPFFER CELLS This incredible magnified behind the liver. image shows a Kupffer cell (yellow) trapping and consuming old red blood cells (red). WOW! Esophagus The liver receives Stomach 2½ pints (1.5 liters) Spleen of blood every minute. Hepatic portal Gall bladder vein BLOOD SUPPLY Small intestine The liver has two blood supplies. The hepatic artery delivers oxygen-rich blood from your heart. The hepatic 135 portal vein carries blood from your digestive organs and supplies 80 percent of the liver’s blood. Nutrients and other substances in the blood are processed by the liver. Large intestine ▶ HEPATIC PORTAL VEIN Veins from many parts of the digestive system, including the stomach and intestines, join together to form the hepatic portal vein, which takes nutrient-rich blood to the liver.

Balanced diet COUNTING CALORIES Eating the same kind of food every day wouldn’t only The amount of energy contained in different be boring; it would be bad for your health. To stay foods is measured in calories. A cheeseburger, healthy and at the right weight for your height you packed with energy-rich fat, contains many more need to eat a balanced diet. This means eating sensible calories than, say, an apple. If you regularly eat amounts of different foods from five main food groups. more calories than you need, the extra energy will be stored by your body as fat and you will become overweight. FUELING THE BODY FIVE A DAY You should try to eat at least five portions of fresh fruits and vegetables every day. Scientists have found that a diet including plenty of fruits and vegetables can help reduce the chances of developing serious diseases, such as cancer and heart disease, later in life. FOOD GROUPS Fruit Proteins To stay healthy, you need to eat the right foods in the right quantities, as shown in the chart here. Eat a lot of grains, such as wheat, since they contain starch, which provides fuel—in the form of glucose—to your body. Vegetables and fruit are rich in minerals and vitamins. These help your body cells work normally. Dairy products contain calcium, needed for healthy bones and teeth. Finally, protein-rich foods provide the building blocks for growth and repair. Grains 136

100 calories 360 calories 650 calories 200 calories 30 calories FUELING THE BODY HOW MUCH ENERGY? Daily intake of calories Age 9–11 12–14 15–17 18+ WOW! 2280 2640 2880 2550 The amount of energy we need Polar explorers who pull each day depends on a number 2050 2150 2150 1940 their own sledges need of factors. Teenagers need more as much as 6,500 calories energy than adults because they per day to survive in the are still growing. Men need more cold, icy conditions. calories than women because, on average, they are bigger and have more energy-burning muscle. FAST OR SLOW? Fast foods, such as burgers, contain large amounts of fats and salt. They are often eaten with soft drinks that contain lots of sugar. It is healthier to eat “slow” foods prepared from a balanced mix of fresh ingredients. Dairy products Vegetables 137

FUELING THE BODY FOOD MARKET This colorful food market in Malaysia shows the wide variety of vegetables eaten as part of the Southeast-Asian diet. Eating lots of fresh fruits and vegetables can help keep you healthy and reduce your chances of developing heart disease and cancer later in life.

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FUELING THE BODY Vitamins WOW! In addition to providing fuel for your body, Not all vitamins come food contains the 13 vitamins that are vital from food. Vitamin D is to health. You need only tiny amounts, but vital for bone growth. One vitamins keep you looking good and feeling form of it is made by your fine—going without them can damage skin when it is exposed your health and cause nasty diseases. to sunlight. WHAT ARE VITAMINS? CHICKEN FEED Each vitamin is a complex natural In the past, many people chemical that has a special job in the in Southeast Asia died body. Vitamins are known by a letter from a wasting disease of the alphabet, and sometimes a called beriberi. In the 1890s, number. Ascorbic acid, for example, is Dutch doctor Christiaan usually called vitamin C, and riboflavin Eijkman found that chickens is vitamin B2. You get vitamins from fed only white rice got the same certain foods in your diet. disease, but those given brown rice were healthy. The brown rice contained Dairy products, vitamin B1, which prevented beriberi. such as milk, are full of Leafy greens, such Citrus fruits are vitamin A. as cabbage and a good source of spinach, contain vitamin C. vitamin K. Eggs are a Chicken and Wheat germ good source other meats contains of B vitamins. contain B vitamins. vitamin E. 140

SICK SCIENCE FUELING THE BODY Eating food rich in vitamin B12 cures a disease called pernicious anemia. In 1928, Dr. William Castle discovered that a protein made by the stomach helps the body absorb the vitamin. He tested this by giving anemia patients vitamin-rich food mixed with liquid strained from his own vomit! Sure enough, they got better more quickly. NIGHT VISION The ancient Romans had a strange cure for people who couldn’t see at night. They roasted a goat and rubbed fat from its liver onto the eyes of the patient, who also ate some of the liver. But it worked! Why? Because liver is rich in vitamin A, which helps you see in the dark. DEADLY VITAMIN Although vitamin A is vital to health, you can have too much of it. The livers of some animals contain so much vitamin A that they are dangerous to eat. The Arctic Inuit people have long known not to eat the livers of polar bears, seals, or husky dogs. However, several European explorers, desperate for food on long polar expeditions, found out the hard way when they became very ill after eating this meat. FAST FACTS CRAZY CORN ■ The best way to make sure you get In the early 1900s, a strange disease swept enough vitamins is to eat a wide variety of across the southern United States. foods, and not stick to just a few favorites. Victims went mad and suffered sores ■ Vitamin C is a vital part of the human on their skin. In 1915, Dr. Joseph diet, but most other animals are able Goldberger found that he could cause to make their own. the symptoms by feeding people ■ Some common foods have vitamins nothing but corn—a staple of the local added to make them more nutritious. diet—and cure them with a dose of yeast ■ Before they are born, babies need extract. He realized the disease was related vitamins supplied by their mothers. to something missing from corn but found in yeast, later identified as vitamin B3. 141

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Every day your kidneys clean WASTE DISPOSAL hundreds of pints of blood and filter out harmful waste chemicals. The wastes are flushed out of your body in urine, along with water your body doesn’t need.

WASTE DISPOSAL Urinary Right kidney The renal system artery carries The inferior vena cava blood into Your body’s cells release returns blood to the kidneys. poisonous waste products. the heart. Many are flushed out of the The renal body by the urinary system. vein carries Two kidneys process your blood out of body’s blood and remove the kidneys. excess water and waste. This Left kidney waste liquid, called urine The aorta carries (pee), is carried to the bladder, blood from and released from the body the heart. via the urethra when you go to the bathroom. The left ureter carries urine from FILLING THE BLADDER the left kidney to the bladder. The kidneys produce urine. It is then pushed The urethra carries by waves of muscle contraction along the urine out of the body. two ureters to the bladder. Urine is stored in the bladder until the bladder sends messages to the brain indicating that it needs to be emptied. Urine then flows out of the body along the urethra. Right ureter ▶ LONG AND SHORT The bladder stores In the female system (shown urine and releases it here), urine is expelled out when convenient. of the body through a short urethra. The male system is the same except that the urethra is longer and passes along the penis. 144

DAILY BREAKS WOW! WASTE DISPOSAL We make up to 2½ pints (1.5 liters) of urine every Your hardworking day—enough to fill half a dozen coffee cups. When your kidneys make up just bladder is full, nerve endings in the bladder’s muscle wall 1 percent of your weight, signal to the brain that it is time for a bathroom break. A sphincter muscle keeps your bladder shut until you are but they consume ready to pee. The sphincter muscle relaxes and the 25 percent of your bladder contracts to help push the urine out. body’s oxygen. OTHER JOBS Your kidneys don’t just filter and clean blood to make urine. They help control your blood pressure to make sure it stays within safe limits. Your kidneys also release a hormone (chemical messenger) that increases the production of red blood cells in your bones. What’s more, they activate vitamin D made by the action of sunlight on the skin. Active vitamin D is needed in the body to make sure that bone-building calcium is absorbed from the small intestine. FAST FACTS KIDNEY MACHINE ■ Every minute of the day, about a quart Sometimes the kidneys stop (liter) of blood passes through your working properly due to kidneys to be filtered and cleaned. disease. One solution is to ■ It takes less than an hour to clean your transplant a healthy kidney entire blood supply. into the body of a person with ■ Each kidney contains around one failing kidneys. But this isn’t million tiny filtration and urine-making always possible. Kidney units called nephrons. dialysis is an alternative ■ In an average lifetime, the kidneys treatment, which cleans a process 12 million gallons (45 million patient's blood by passing it liters) of blood, enough to fill 18 through a machine that acts Olympic-size swimming pools. like an artificial kidney. 145

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WASTE DISPOSAL TESTING THE WATER In the past, doctors used uroscopy—or urine gazing—to help them diagnose diseases, as shown in this 15th-century woodcut. They would examine the color, smell, and cloudiness of a patient’s urine. Sometimes doctors would also taste it!

WASTE DISPOSAL Waterworks For your body to work at its best, the substances that make up your blood have to be carefully controlled. Your kidneys play a key role in this. All through the day and night, they remove excess water, salts, and poisonous wastes from your blood to make urine. The cleaned blood is then returned to your body’s blood supply. INSIDE A KIDNEY The cortex is the outer part A network of blood vessels delivers blood to of the kidney. and from the cortex (outer part) and the medulla (inner part) of your bean-shaped The hollow kidneys. These areas are where urine is renal pelvis made. Urine then drains into the renal collects urine. pelvis, a flattened, funnel-like tube, which empties it into the ureters for the next part of its journey out of the body. The renal artery delivers the blood that has to be cleaned. The renal vein removes the cleaned blood. ▲ BLOOD FILTERS The ureter The medulla is A microscopic view of the kidney carries urine the inner part reveals the tiny nephrons that away from of the kidney. loop between the kidney’s cortex the kidney. and medulla. They produce urine by filtering blood. 148