Human Body
 

[B][SIZE="5"]Human Body[/SIZE][/B]

Your body is amazing. Did you know you have more than 200 bones and 600 muscles? Your nerves carry messages from your brain to make those muscles cooperate so you can stand up and move around. Your blood vessels could stretch all the way around the planet! White blood cells stand guard like soldiers waiting to attack any invader. Your heart, lungs, stomach, and other organs are at work 24 hours a day for your entire life. There are too many parts inside you to count, but they all work together to keep you alive. No machine is as complex as you are.

The many parts of your body are grouped into systems. Each system has a job to do in your body. The systems work together to keep you alive and healthy.

BONES AND MUSCLES

The bones and muscles of your body let you move around. Tough bands called ligaments connect your bones to each other. The connections are called joints. Some joints can move a lot. Your arm at your shoulder joint can move in circles. Your lower leg at your knee joint can only move back and forth. The bones in your skull have special joints that cannot move at all.

Muscles attached to bones pull on them to make your body move. The muscles get their orders to move from your brain and nerves.

THE NERVOUS SYSTEM

Your brain and nerves make up your nervous system. Your brain is the command center of your body. Your brain sends signals through your nerves. Some signals from your brain control your muscles. Suppose you want to walk across the street. Your brain sends signals that tell the muscles in your legs to move.

You do not have to think about some of the signals your brain sends out through your nerves. Your nervous system tells your heart to beat and your lungs to breathe even when you are sleeping.

Nerves also send signals back to your brain. Nerves tell your brain what your eyes see. They tell your brain when you stub your toe.

CIRCULATORY SYSTEM

Your circulatory system is made up of your heart and blood vessels. Blood vessels are flexible, hollow tubes. Your heart pumps blood through blood vessels. It sends blood to your lungs to pick up oxygen. It pumps blood out to all parts of your body.

Arteries are blood vessels that carry oxygen-rich blood out to your body. Veins are blood vessels that carry blood back to your heart. The blood vessels near your heart are thick. Farther from your heart, the blood vessels are smaller.

The tiniest blood vessels are called capillaries. Capillaries go all through your body. They give up oxygen and nutrients that your body needs. They carry away waste products.

IMMUNE SYSTEM

Your immune system defends against germs and other things that make you sick. White blood cells and other chemical weapons of the immune system rush to find and destroy the germ. Special white blood cells and chemical “watchdogs” called antibodies stand guard. Sometimes antibodies grab onto a germ that shows up. White blood cells called T cells attack germs directly.

Many T cells get stored in little pouches called lymph nodes. Lymph nodes in your neck and other places sometimes swell up when your body is fighting off germs. Some people call this “swollen glands.”

RESPIRATORY SYSTEM

Respiration is breathing. You use your lungs to breathe. You breathe air into your lungs. The air contains oxygen, a gas you need in order to live. Blood in your lungs picks up the oxygen and carries it to all parts of your body.

Blood coming back to your lungs gives off carbon dioxide, a waste gas. Your lungs send carbon dioxide out of your body when you breathe out.

DIGESTIVE SYSTEM

Your digestive system is like a long tube that goes down through your body. Your digestive system breaks down the food you eat. It breaks down food so that your body can use it for energy.

Your teeth grind up food in your mouth and mix it with saliva. You swallow the ground-up food. It goes into your stomach where it gets broken down even more.

Food goes from your stomach to your small intestine. Nutrients pass through the walls of your intestine and into your blood. Your blood carries the nutrients to all parts of your body.

Your body gets rid of any leftover waste products. Liquid waste products go to your kidneys. You get rid of these waste products as urine. Solid waste products go to your large intestine. You get rid of these waste products as feces.

OTHER SYSTEMS

Your body has other systems. One is the reproductive system, which differs in males and females. The male reproductive system makes sperm. The female reproductive system makes eggs. An egg fertilized by sperm grows into a baby.

The endocrine system is made up of glands. The pituitary gland under the brain is the master gland of the body. It controls the activities of other glands. Glands control how your body burns food for energy. They control how fast you grow and do many other important things in the body.

Cell
 

[B][SIZE="5"]CELL[/SIZE][/B]

All plants and animals are made of cells. Most cells are much too small to see. It takes millions of cells, for example, to make just one of your fingers. Some tiny creatures, such as bacteria, are made of only one cell.

If you want to see what a cell looks like, you could use a microscope. Or, you could get an egg. A bird’s egg is actually a single, giant cell. The eggs you eat for breakfast come from chickens.

AN EGG IS A CELL

An egg is a very big cell. Take a look at an egg that has been broken into a bowl. One of the first things you see is a yellow center. You call this center the yolk. Biologists call it the nucleus of the cell. Almost all cells have a nucleus.

The genes of the cell are in the nucleus. The genes in the chicken egg tell what the bird will look like. The genes tell the cell how to do its work. Different plants and animals have different genes.

The clear, jelly-like stuff around the nucleus is called cytoplasm. Much of the work of the cell goes on in the cytoplasm. This work keeps plants and animals alive.

A chicken egg is a special kind of cell. It has a hard shell to protect the egg. Most cells are much smaller and do not have a hard shell.

WHAT ARE THE KINDS OF ANIMAL CELLS?

There are many kinds of animal cells. The different parts of your body, such as your muscles, skin, and brain, are made up of different types of cells. The cells perform different functions. Muscle cells help you move. Skin cells keep harmful things from getting into your body. Brain cells allow you to think.

Animal cells come in all sizes. Some are so small that 10,000 of them would be only as wide as a single hair. The ostrich egg is a cell that is as big as a baseball.

WHAT DO ANIMAL CELLS LOOK LIKE?

Animal cells come in many different shapes. Skin cells are flat. Red blood cells are round like wheels. Muscle cells are long and thin. Nerve cells have long arms and look like octopuses.

Cells have a kind of framework. The framework is called a cytoskeleton. The cytoskeleton gives an animal cell its shape. It holds the insides of the cell in place.

THE INSIDES OF ANIMAL CELLS

Cells have organs just like you have organs. The organs of a cell are called organelles. Organelles do all the things a cell needs to stay alive and to perform its function in the body.

Each type of organelle has a different job. The nucleus is the organelle that holds the genes. The genes carry a code that tells the cell what to do. Genes tell a cell when to divide.

The cytoplasm contains organelles that do other jobs. Organelles called mitochondria make energy for the cell. Muscle cells have lots of mitochondria because they use a lot of energy. Proteins, chemicals that cells need to survive, get made in organelles called the endoplasmic reticulum and the Golgi apparatus. Lysosomes are small, rounded organelles that act as the cell’s recycling center and garbage disposal.

Animal cells are surrounded by a covering called the cell membrane. The cell membrane lets some materials pass into and out of the cell. It keeps other, harmful materials out.

WHAT DO PLANT CELLS LOOK LIKE?

Plant cells are square or rectangular in shape. Plant cells have a nucleus and many of the same organelles that animal cells have. In addition, plant cells have a sturdy cell wall that surrounds the cell membrane. Plant cells also have a big bag inside called the central vacuole. The central vacuole stores water and takes up most of the cell. Green plant cells contain small green spheres called chloroplasts.

THE INSIDES OF PLANT CELLS

Chloroplasts are organelles that absorb sunlight and use its energy to make food for the cell. The job that chloroplasts do is called photosynthesis.

The central vacuole stores water filled with salts, sugars, and other nutrients. The vacuole swells up when it’s full of this water. The full vacuole gives plants sturdy stems and plump leaves and flowers. Without enough water inside, the cells collapse and the plant wilts. In flowers, the material in the vacuole gives the flower its color. The central vacuole also collects plant wastes.

The strong cell wall gives a plant cell its rigid shape. The wall lets the central vacuole swell up without bursting the cell.

HOW DO CELLS MULTIPLY?

All cells of living things come from other cells. They come from cells that divide.

There are two steps in cell division for plants and animals. First, the nucleus divides in two. Then, the cytoplasm divides and the cell splits into two cells, each with its own nucleus.

There are two ways a cell can divide. The nucleus of most cells divides by a process called mitosis. Mitosis makes ordinary cells such as skin or blood cells. The two cells produced by mitosis are like identical twins. Each has a full set of genes.

The nucleus of a sex cell divides by a different process called meiosis. Cells produced by meiosis have only a half set of genes. These sex cells, such as sperm and egg cells, can together produce whole new animals. When a sperm fertilizes (combines with) an egg, the new cell has a full set of genes. Half of the genes come from the father’s sperm cell and half from the mother’s egg cell. That’s how babies get genes from both parents.

Genes and Heredity
 

[B][SIZE="5"]Genes and Heredity[/SIZE][/B]

Have you ever heard a news reporter talk about DNA? Reporters talk about DNA found at the scene of a crime. They talk about police finding DNA “fingerprints.” Police sometimes use DNA as a clue to find out who committed the crime.

WHAT IS DNA?

DNA is a substance that makes up genes. Everything alive has genes. Plants have genes. Animals have genes. You have genes.

Genes are the basic units of heredity. Heredity means all the characteristics you inherit from your parents. You get your genes from your parents. You inherit half of your genes from your mother. You inherit half of your genes from your father.

Genes are a kind of code. A tree’s genes tell what shape its leaves will be. A cat’s genes tell what color its fur will be. Your genes tell what color your eyes will be. Your genes tell what color your hair will be. Everything about you comes from the code in your genes.

WHERE ARE GENES?

Genes line up on strands called chromosomes in cells. Everything alive is made up of cells. Chromosomes are in the center, or nucleus, of cells.

Different parts of you are made of different kinds of cells. Your muscles are made of muscle cells. Your skin is made of skin cells. The code in your genes tells your body to make different kinds of cells. The genes in each cell tell the cell how to work. They tell the cell when to make new copies of itself.

WHO DISCOVERED GENES?

An Austrian monk named Gregor Mendel first saw inherited patterns in pea plants. He experimented with pea plants in the 1860s. One of the things, or traits, Mendel studied was what makes some pea plants tall and some short. He said that the traits must come from units of heredity passed from the parent plants. These units were later called genes.

In the mid-1900s, scientists discovered that genes are made of DNA. In the 1970s, scientists learned how to change DNA with genetic engineering. Scientists also learned that problems with certain genes cause diseases. Muscular dystrophy, cystic fibrosis, and hemophilia are some genetic diseases—diseases caused by problems in genes. Today, scientists are looking for ways to cure genetic diseases by altering genes through a process called gene therapy.
d.

Bones and Skeleton
 

[B][SIZE="5"]Bones and Skeleton[/SIZE][/B]

Squeeze your arm. The outside of your arm is soft, but there is a hard part inside. The hard part is a bone. There are bones in your arms and in your legs. Bones go up the middle of your back. They go around your chest. All of your bones together make up your skeleton. Your skeleton holds your body up. It gives your body its shape. Bones do many other important jobs in your body.

WHAT DO BONES DO?

Many bones protect the soft parts inside your body. Skull bones around your head protect your brain. Rib bones make a cage around your chest. Your rib cage protects your lungs and heart.

Muscles hook on to bones. Muscles pull on your bones to make them move. Muscles and bones together let you stand, sit, and walk around.

Blood is made in the center of bones. The center of a bone is filled with bone marrow. Bone marrow is soft. Red and white blood cells are made by bone marrow. Red blood cells carry oxygen to all parts of your body. White blood cells help your body fight germs.

Three tiny bones help you hear. The three bones are deep inside your ears. One of these bones is called the stirrup bone. It is the smallest bone in your body.

WHAT ARE BONES MADE OF?

There are two kinds of bone. One kind is called compact bone and the other is called spongy bone. Compact bone is the hard and smooth part on the outside of a bone. The long bones in your arms and legs have lots of compact bone. Spongy bone usually lies under the compact bone. Spongy bone is at the ends of arm and leg bones as well. Bones of the pelvis (hipbone), ribs, breastbone, backbone, and skull also contain spongy bone.

Your skeleton also contains cartilage. Cartilage is like bone but softer. It bends easily. There is cartilage in body parts that must be tough but able to bend. There is cartilage in the tip of your nose and in the outer part of your ear.

WHAT ARE JOINTS?

Joints are the places where two or more bones meet. Most bones are tied together at joints by tough bands called ligaments.

Different kinds of joints let you move in different ways. Move your lower arm up and down. Keep your upper arm still. The joint that joins your upper and lower arm is called the elbow. Your elbow works like a hinge. It lets you move your lower arm, but only up and down. Now swing your arm all around from your shoulder. A joint in your shoulder called a ball-and-socket joint lets you move your arm in many directions.

Your skull is made of many bones that do not move. They are held together in one solid piece by suture joints.

HOW DO BONES GROW?

Bones grow or change as long as you live. Your head and other parts of your skeleton had a lot of cartilage when you were born. Bones replaced the cartilage as you got older.

Bones get thicker and longer as you grow taller. Bones keep growing in teenagers. Bones stop growing longer in adults.

Some bones join together as you get older. Your skeleton had more than 300 bones when you were first born. An adult has 206 bones. The longest and strongest bone in adults is the thighbone, in the upper leg.

Bones are replaced a little bit at a time even after they stop growing. This replacement goes on for as long as you live. Your body needs a mineral called calcium to keep strong bones. Milk has lots of calcium. Running and other exercise also helps build strong, thick bones. Some older people have thin, weak bones. Their bones can break easily. Getting enough calcium and exercise can help keep bones from getting weak and thin.

WHAT HAPPENS TO BROKEN BONES?

Sometimes people have accidents that break bones. Maybe they fall out of a tree or down a flight of stairs. Sometimes football players or other athletes break bones when they are playing sports.

A doctor has to fix a broken bone. First, an X-ray picture shows the doctor what the broken pieces of bone look like. Then, the doctor fits the broken parts of the bone back together. This is called setting the bone. Sometimes a broken bone must be put back together with wires or pins.

A broken bone should not be used until it is healed. The doctor makes a hard case called a cast for an arm or leg with a broken bone. New bone starts to grow around the break. The pieces grow together and heal the broken bone.

Brain and Nervous System
 

[B][SIZE="5"]Brain and Nervous System[/SIZE][/B]

What kind of supercomputer can write stories, do math problems, draw pictures, play games, see through eyes, hear someone talking, talk back, and network with devices that make snacks in the microwave oven? Your brain and nervous system can do all these things. Do you think a computer will ever be as powerful as your brain?

You think with your brain. Your brain also sends signals through a network called your nervous system. It tells your legs to walk and run. It tells your hands and arms to put popcorn in the microwave. You don’t even have to think about many of the things your brain does. Your brain tells your heart to beat. It tells your lungs to breathe in and out, even when you are sleeping.

Your brain also controls your feelings. Such feelings as joy, sadness, love, anger, and fear all come from your brain.

WHAT IS MY BRAIN MADE OF?

Your brain is made of about 100 billion nerve cells. It looks like a lump of pinkish-gray jelly. The surface of the brain is wrinkled, and deep grooves divide it into sections. A network of blood vessels brings oxygen and food to your brain cells and carries away wastes. Your brain is protected by bone called your skull. Liquid and skinlike tissues also protect your brain.

When you were born, your brain weighed about • pounds (about 0.35 kilograms). Your brain keeps on growing while you grow up. By the time you reach the age of 20, your brain will weigh about 3 pounds (1.3 kilograms).

Your brain has three main parts. The parts are called the cerebrum, the cerebellum, and the brain stem. The cerebrum makes up the largest part of the brain. The cerebellum is underneath the back part of the cerebrum. The brain stem connects with the spinal cord at the bottom of the brain.

Your cerebrum and cerebellum are divided into two parts. These parts are called the right brain and the left brain. The right side of your brain controls the left side of your body. The left side of your brain controls the right side of your body. Nerves from the right and left side of your body cross over when they enter your brain.

WHAT DOES THE CEREBRUM DO?

Your cerebrum makes up most of your brain. Your cerebrum solves problems and makes wishes. All of your thinking goes on in your cerebrum. Speech, language, and emotions come from your cerebrum, especially your cerebral cortex. The cerebral cortex is the outer part of the cerebrum.

Your cerebrum also gets signals from your senses. Nerves carry the signals. Nerves from your eyes and ears go to parts of the cerebrum that let you see and hear. Nerves carry signals to your cerebrum that let you feel, smell, and taste.

Your cerebrum sends messages out along nerves. The messages tell your legs to walk or run. They tell your arm and hand to wave when you see a friend across the street.

WHAT DOES THE CEREBELLUM DO?

Your cerebellum coordinates and fine-tunes your body movements. Your cerebrum might tell your hands and arms to hit a baseball. Your cerebellum controls how you swing the bat and make contact with the ball.

Your cerebellum helps your fingers play the piano, guitar, or violin. It helps you keep your balance when you run, jump rope, or walk along a curb.

WHAT DOES THE BRAIN STEM DO?

Your brain stem takes care of all the things that you do but don’t need to think about doing. It keeps your heart pumping blood. It keeps your lungs breathing air. It makes your eyes blink. It pulls your hand back really fast if you touch a hot pot on the stove.

WHAT IS THE NERVOUS SYSTEM?

Your nervous system consists of the brain, spinal cord, and nerves that run throughout your body. The nervous system carries messages to your muscles and organs. These messages tell your body what to do.

Your spinal cord is made of bundles of nerves. It starts in your neck and goes down your back. Nerves go out from the spinal cord to other parts of your body. Nerves from the spinal cord extend to the tips of your fingers and toes. Your spine, or backbone, protects your spinal cord.

Heart and Circulation
 

[B][SIZE="5"]Heart and Circulation[/SIZE][/B]

Did you ever send a valentine with the shape of a heart on it? Did you ever hear someone say, “That came straight from my heart?” People talk about hearts a lot. People have always known that hearts are very important.

You have a heart. Your heart does not look like a valentine heart. Your heart is a pump. When you run very fast, your heart pumps hard and fast. You can feel your heart pumping, or beating.

WHAT DOES A HEART LOOK LIKE?

Your heart looks like an upside-down pear. It is about the size of your closed fist. It is almost in the middle of your chest. It is just off to the left side.

Your heart is made of muscle. It is divided into four parts called chambers. The chambers are hollow inside. The two chambers on top are called atria. The chambers on the bottom are called ventricles. Your heart also has four valves that let blood in and out of the chambers.

Tubes called arteries come out of your heart. Tubes called veins go into your heart. Arteries and veins are also called blood vessels.

THE HEART PUMPS BLOOD

Your heart pumps blood. Blood comes into the atria or top chambers of your heart. Your ventricles, or bottom chambers, pump blood out to every part of your body.

Blood going out of your heart carries food and oxygen. Every part of your body needs food and oxygen for energy. You need energy for your body to work and for you to stay alive. Your heart pumps blood carrying food and oxygen through your arteries. Big arteries carry the blood to your legs and arms. The arteries get smaller and smaller the farther out they go. Little blood vessels called capillaries take blood to your cells. Everything in your body is made of tiny cells.

Your cells give off waste products when they make energy from food and oxygen. One of these waste products is a gas called carbon dioxide. The blood in your capillaries picks up the waste products. Capillaries connect to bigger veins. The pumping of your heart pushes the blood through your veins.

WHAT HAPPENS TO THE BLOOD IN VEINS?

Your veins carry blood back to your heart. The chambers on the right side of your heart take care of blood coming back through your veins. First, the blood comes into your right atrium, the top chamber. Your right atrium pumps the blood into your right ventricle, the bottom chamber. Your right ventricle pumps the blood through an artery into your lungs.

WHAT HAPPENS TO BLOOD IN THE LUNGS?

Your blood has to get rid of carbon dioxide. It has to get a fresh supply of oxygen. Your lungs take care of both jobs. Carbon dioxide from your blood goes into your lungs. Your lungs get rid of the carbon dioxide when you breathe out.

Then you breathe in. Your lungs get oxygen from breathing in air. Your lungs fill up with oxygen. Your blood picks up a new supply of oxygen from your lungs. Now your blood is ready to go out through your arteries to all the parts of your body.

The chambers on the left side of your heart take care of blood going out through your arteries. Special veins send blood from your lungs to your left atrium, or top chamber. The blood goes from the left atrium to the left ventricle. The left ventricle pumps the blood out through your arteries to every part of your body.

HOW DOES THE HEART PUMP?

Make a fist. Open your fist slightly, and then squeeze it closed. Open and close your fist again and again. This is sort of how your heart pumps blood. The muscles in your heart squeeze the chambers.

To open and close your fist, you have to think about doing it. You don’t have to think about squeezing your heart muscles. Your brain tells your heart to pump over and over again. Your heart pumps when you are awake. Your heart pumps when you are asleep. Your heart pumps faster when you run fast. Your body needs more oxygen when you run.

Your heart is better than any pump made. It beats over and over again, day and night. The heart of a 76-year-old person has beat nearly 2.8 billion times. It has pumped about 179 million quarts (169 million liters) of blood. No one can live if their heart stops beating for more than a few minutes.

HOW TO KEEP THE HEART HEALTHY

Hearts can get sick. The special arteries that bring blood and oxygen to the heart muscle can clog up. They can clog up with clumps of fat called plaque. Blood cannot flow through clogged arteries. Clogged arteries can cause heart attacks.

Exercise is one way to keep your heart healthy. Eating fruits, vegetables, and low-fat meats is another way to keep your heart healthy. You should have regular physical checkups. The main way to keep your heart healthy, however, is not to smoke.

Lungs and Breathing
 

[B][SIZE="5"]Lungs and Breathing[/SIZE][/B]

Take a deep breath. Feel how your chest swells. Lungs inside your body are filling up with air. Every time you breathe in, or inhale, your lungs fill up with air. When you breathe out, or exhale, your lungs empty out. You probably breathe about 20 times every minute. If you are running or playing hard, you might take 80 breaths a minute. What happens to all this air?

It goes to your lungs. Your lungs are like two big sponges inside your chest. Lungs are the body organs you use for breathing. All mammals, birds, reptiles, most amphibians, and even some fish have lungs.

Your lungs are inside a big cave in your body called your chest cavity. One lung is on the right side of your chest cavity. The other is on the left side. Your heart is in between your lungs. The sides, or walls, of this cavity are made of thick muscles and bones called ribs. The muscles make the walls move out and in when you inhale or exhale. This makes your lungs fill up or empty out. The muscles and ribs also protect your lungs from getting hurt.

AIR TAKES A TRIP

When you inhale air through your nose or your mouth, that air takes off on an incredible journey. First it goes down the back of your throat, past your voice box, and into your windpipe, or trachea. Your trachea is like a tunnel that branches off into two more tunnels called bronchial tubes or bronchi. Each bronchial tube goes off to one of your lungs. It doesn’t matter whether the air goes to the right lung or the left lung. Both lungs do the same job.

Inside your lungs, the bronchi are like upside-down trees. They split into smaller and smaller branches and then into many twigs. The twigs end in tiny air sacs called alveoli. So far, the air has just been along for the ride. Inside the alveoli the air really goes to work. Its job is to keep you alive.

The job of the air in your alveoli is to bring oxygen into your body and take carbon dioxide out. Oxygen and carbon dioxide are two invisible gases in air. The secret to doing this job is blood.

THE TRIP OUT

The air in your alveoli sends oxygen into your blood. Your blood flows through a system of tubes called blood vessels. First, blood full of oxygen goes to your heart. Your heart is like a big pump. It sends the oxygen-filled blood whooshing off to all parts of your body.

The blood vessels that deliver oxygen are called arteries. Arteries branch off into smaller and smaller tubes. The tiniest blood vessels are called capillaries. Capillaries reach every cell in your body and drop off the oxygen. In your cells, the oxygen works with food to make energy for you.

THE TRIP BACK

After your blood drops off the oxygen in your body’s cells, it picks up carbon dioxide. Carbon dioxide is a waste product. It is created when your body cells use oxygen and food to make energy. This waste has got to go. So your blood carries it back to your lungs.

The blood vessels that carry blood full of carbon dioxide are called veins. Veins carry blood back to your heart. Your heart pumps the blood into your lungs. Finally, your blood drops off the carbon dioxide in your alveoli.

The carbon dioxide leaves the opposite way that the oxygen came in. Your lungs push carbon dioxide out of your body when you exhale. Taking oxygen into your body and giving off carbon dioxide is called respiration.

The story gets even better. Plants use the carbon dioxide that you and other animals exhale. Carbon dioxide and sunlight help plants make food. A waste product that plants give off is the oxygen that you need for life.

KEEPING YOUR LUNGS HEALTHY

Germs can cause lung diseases such as tuberculosis and pneumonia. These are serious health problems. Smoking tobacco causes other lung diseases, such as lung cancer and emphysema. People with emphysema have trouble getting enough oxygen in their blood. Diseases caused by smoking can be deadly. Smoking also makes asthma and chronic bronchitis worse. These conditions make breathing difficult.

It is very important to keep your lungs healthy. The best thing you can do for your lungs is to not smoke.

Digestive System
 

[B][SIZE="5"]DIGESTIVE SYSTEM[/SIZE][/B]

You eat when you get hungry. It seems so simple. You take a bite out of a sandwich.

That bite then begins an amazing journey. That bite goes through every part of your digestive system. Your body digests, or breaks down, the food into smaller and smaller parts. At the end of the journey your blood carries chemicals from your sandwich to every part of your body.

WHAT IS YOUR DIGESTIVE SYSTEM LIKE?

Your digestive system is one long, winding tube. The digestive system of a grown-up is 20 to 30 feet (6 to 9 meters) long.

Food goes from your mouth to your throat. It then slides down a tube called the esophagus. It drops into your stomach. From there it gets squeezed into your small and large intestines.

WHERE DOES DIGESTION START?

You start to digest food in your mouth. You bite off a piece of sandwich. Your teeth grind the bite up. Your mouth waters with a liquid called saliva. Saliva helps make the bite of chewed-up sandwich soft and wet.

After you chew up the bite of sandwich, you swallow it. Your tongue pushes the chewed-up food into your esophagus. Your esophagus is like a chute that sends the food into your stomach.

WHAT HAPPENS IN YOUR STOMACH?

Your stomach breaks down the food even more. Your stomach is like a bag made of muscles. Liquids called digestive juices pour into your stomach. Your stomach muscles churn the food around to mix it with the juices. The juices break down the food. The food turns into a liquid in your stomach.

That bite of sandwich does not look like bread, lettuce, cheese, or meat anymore. It is broken down into chemicals called fats, proteins, starches, and sugars.

It takes your stomach about four hours to do its job. Your stomach then sends the liquefied food on to your small intestine. You start to feel hungry again when your stomach is empty. Sometimes your stomach muscles start to churn when your stomach is empty. When this happens, you can hear your stomach growl.

WHAT HAPPENS IN YOUR SMALL INTESTINE?

The job of digesting your sandwich gets finished in your small intestine. Your small intestine is a long and narrow, twisty tube. The small intestine is by far the longest part of the digestive system. Muscles surrounding this tube push the liquid along. More juices break the food down even further.

Eventually the food is broken down into chemicals that your body can use. These chemicals go through the wall of your small intestine. They go into tiny blood vessels just outside the wall. Your blood picks up the chemicals. Your blood carries them to every part of your body. The food chemicals go from your blood into your cells. Your cells use the chemicals to make the energy you need to do your homework, run, and play.

Other organs around your small intestine help it digest food. Your liver and gallbladder help digest fats in foods. Your liver also helps your body store extra food that it cannot use right away. Your pancreas creates the chemical insulin. Insulin helps your body use sugar.

There are some leftovers that get to the end of your small intestine. The leftovers go into your large intestine.

WHAT HAPPENS IN YOUR LARGE INTESTINE?

Your large intestine is shorter and thicker than your small intestine. Your large intestine takes water out of the leftovers. It also takes out some vitamins and minerals.

Bacteria that live in your large intestine break down any leftover food. Bacteria are tiny living things that you can only see with a microscope. What is left in your large intestine is solid waste.

Muscles in your large intestine push the waste along. It goes from your colon to your rectum. The waste gets pushed out of your body through an opening called the anus.

Muscles
 

[B][SIZE="5"]MUSCLES[/SIZE][/B]

How strong are you? Can your legs run fast? Can your arms lift heavy books? Your muscles make your legs run fast. Your muscles let your arms lift heavy books. Without muscles, you wouldn’t be able to move at all!

HOW DO MUSCLES WORK?

Muscles work by tightening and loosening. Tightening is called contraction. Loosening is called relaxing. Your nerves tell muscles when to contract.

Suppose you see a cookie on a table. You want to pick up that cookie and eat it. Your brain sends out a signal. Nerves carry the signal from your brain to your arm and hand muscles. The signal tells muscles in your arm to contract. Your arm reaches out for the cookie. Then the signal tells muscles in your hand to contract and grab the cookie. Muscles in your arm contract to bring the cookie to your mouth. Your hand pops the cookie into your mouth. Your jaw muscles contract and relax so you can chew the cookie. Yum.

WHAT ARE MUSCLES MADE OF?

Muscles are bundles of thin strands called fibers. The muscle fibers are made of substances called proteins.

There are two types of muscle fibers. The two types are slow twitch and fast twitch. Your fast-twitch muscle fibers contract rapidly. These fibers give you bursts of power. When you suddenly jump or run fast while playing tag, your fast-twitch muscle fibers are hard at work.

Slow-twitch muscle fibers allow you to keep doing exercises. They give you endurance. When you run a long way, your slow-twitch muscle fibers are doing most of the work. Some kinds of muscle have both fast-twitch and slow-twitch fibers.

ARE THERE DIFFERENT KINDS OF MUSCLES?

There are three types of muscle called skeletal, smooth, and cardiac. Each kind of muscle has a different job to do.

Your skeletal muscles are attached right to your bones. These muscles contract and relax to make your bones move. These are the kind of muscles you use to run or swim or reach for cookies. You usually have to think about making these muscles contract.

Smooth muscles move automatically. You do not have to think about your smooth muscles to make them work. Your digestive system is surrounded by smooth muscle. Smooth muscles push food through your digestive system. Your biggest blood vessels are surrounded by smooth muscle to make them stronger.

Your cardiac muscle makes your heart beat. You do not have to think about moving your cardiac muscle. Your cardiac muscle contracts automatically.

WHAT MAKES MUSCLES STRONG?

Exercise makes muscles stronger. Lifting heavier and heavier weights makes your skeletal muscles stronger. Many people lift weights to make their arm and leg muscles stronger. A gym teacher or special trainer can show you how to lift weights safely.

Running, walking, swimming, and jumping rope are exercises that can make your heart muscle stronger. Any exercise that makes your heart beat faster makes your heart muscle stronger.

Muscles can get weaker if they are not used. Using your muscles every day keeps them strong and healthy

Eye
 

[B][SIZE="5"]EYE[/SIZE][/B]

Think about all the things you do with your eyes. You watch TV and read books. You surf the Internet. You keep your eye on the ball when you play sports. You see your family and friends. Your eyes are your windows on the world.

Your eyes are like cameras that focus on what is in front of you. Your eyes work together with your brain to create a picture of the world. The process of creating the picture starts when light rays enter your eyeball.

LOOKING AT YOUR EYEBALL

Look at one of your eyes in a mirror. Your eyeball is round. The inside of your eye is a transparent (see-through), jellylike material called the vitreous humor. The vitreous humor gives your eyeball its shape. You can’t see much of the vitreous humor because it is surrounded by an outer part, or wall. The wall of your eyeball is made up of outer, middle, and inner layers.

THE OUTER WALL

The outside layer of your eye is a protective coating called the sclera and the cornea. The sclera is the white part of your eye. The cornea is clear and goes over the center of your eye, the part you look through. Light rays enter your eye through your cornea.

THE MIDDLE WALL

The middle layer of the wall has three parts called the iris, the ciliary body, and the choroid. What color are your eyes? The color comes from your iris. Your iris is in the center of your eye. It can be shades of brown or blue. The black circle in the center of your iris is called the pupil. It gets bigger or smaller to control how much light comes into your eye.

The ciliary body goes around your iris and connects to the lens of your eye. Muscles in the ciliary body pull on the lens to focus it. The blood vessels that bring blood to your eye are also part of the middle wall. These blood vessels are called the choroid.

THE INNER WALL

The inside layer of your eyeball wall is called the retina. Your cornea and lens focus light rays on your retina just as a camera lens focuses light rays on film. They bend light rays coming into your eye so that they will strike the center of the retina called the macula lutea. This is where you have your sharpest vision.

RODS AND CONES

Your retina has millions of light-sensitive cells called rods and cones. These cells pick up the tiniest dot of light that gets to your retina. There are bits of colored material called pigment in the rods and cones. Pigment in the rods lets you see shades of gray and helps you see at night. Pigment in the cones lets you see colors.

The rods and cones change light rays into electrical signals. Nerves pick up these signals and carry them to your optic nerve.

OPTIC NERVES

An optic nerve leads from each of your eyes to your brain. Each eye picks up slightly different images. When these images get put together, you can see in 3-D. You have depth perception that lets you tell how far away things are.

Your optic nerves are like big cables that carry all the signals to a special part of your brain. This “media center” in your brain makes a picture of the world. It gives you sight.

EYE PROTECTION

Your eyeballs are set into two holes in your skull called eye sockets. The bones of your skull protect your eyes. Muscles let you turn your eyes in their sockets.

Eyelids and eyelashes also protect your eyes. You can close your eyelids to keep dust or bright light out. Your eyelashes are a fringe of short hairs on each eyelid. They screen out dust when your eyelids are partly closed.

Inside the eyelid is a thin layer called the conjunctiva. It covers part of the sclera. Each eye also has a tear gland that gives off salty liquid to wash small particles out of your eye.

VISION PROBLEMS

Do you wear contacts or glasses? If not, you probably know someone who does. Many people need contacts or glasses because they are nearsighted. Things far away look blurry. Light rays focus in front of the retina because the eyeball is too long.

Some people have the opposite problem. They are farsighted and can’t see close-up things very well. In farsightedness, light rays focus behind the retina because the eyeball is too short.

Astigmatism is another vision problem. A person has an astigmatism when their cornea is unevenly curved. Older people sometimes need reading glasses because the muscles in their eyes can no longer focus on things that are nearby.

EYE DISEASES

Diseases strike different parts of the eye. A sty is an infection of the eyelid. Conjunctivitis, or pinkeye, is an infection of the thin layers covering the eyelid and outer eyeball.

Many eye diseases are most common in older people. Sometimes the lens of the eye gets cloudy over time. This condition is called cataracts. Retinas can get detached (come loose) from the back of the eye and cause blindness. Glaucoma occurs when fluid gets trapped between the cornea and the lens and puts pressure on the eye. A problem called macular degeneration affects the center of the retina. It can cause blindness in older people.

There are many treatments for eye diseases. Doctors treat infections with drugs. They use laser beams to weld detached retinas back into place. Surgeons can replace clouded lenses with clear plastic ones. They can also replace diseased corneas.

It is very important to protect your eyes. Get regular eye examinations. Wear eye protectors when doing dangerous work or playing rough sports. Wear sunglasses that protect against harmful rays from the Sun. Your eyes are too important to take chances with!

Ears and Hearing
 

[B][SIZE="5"]Ears and Hearing[/SIZE][/B]

You have an ear on each side of your head. Your ears let you hear sounds. You can hear music and other nice sounds. You can hear sirens and other warning sounds.

Your ears do something else very important. They help you keep your balance. You can walk and ride a bike without falling over because of your ears.

HOW DOES SOUND WORK?

Anything that moves back and forth makes sound. Moving back and forth is called vibrating. Pluck a guitar string and watch it vibrate back and forth. The vibrations make sound waves.

Sound waves are a lot like water waves. If you throw a pebble in a lake or pond, you can see the waves move out in circles. Sound waves move out in circles from whatever is vibrating.

Most sound waves you hear travel through air. Sound waves can also travel through water and even through solid things. You can hear your own voice because your solid skull bones vibrate.

Your ears pick up the sound waves. Your ear has three main parts that let you hear. The parts are called the outer ear, middle ear, and inner ear.

THE OUTER EAR

Your outer ear is the part that sticks out on the side of your head. The outer ear collects sound waves. The sound waves travel down a tube to your eardrum. Your eardrum is a thin layer that separates the outer ear from the middle ear. The sound waves make your eardrum vibrate.

THE MIDDLE EAR

Your middle ear is inside your head on the other side of your eardrum. Your middle ear has three tiny bones. Your vibrating eardrum makes the bones move. The bones carry the vibrations to your inner ear.

Your middle ear is filled with air. A tube connects your middle ear with your nose and throat. This tube helps keep too much air from building up inside your ear. The tube makes your ears pop in an elevator or airplane.

THE INNER EAR

Your inner ear has many parts and tubes that twist and turn. A part called the cochlea is very important for hearing. The cochlea looks like a snail shell. It is filled with fluid and tiny hairs. The moving bones in your middle ear make the fluid and tiny hairs in the cochlea move.

The tiny hairs link to nerves. The nerves carry signals to your brain. Your brain tells you what the sound is. It tells whether you are hearing a train whistle or a bird singing.

HOW DO EARS HELP WITH BALANCE?

Your sense of balance comes partly from your inner ear. Fluid, tiny hairs, and small grains of minerals in your inner ear work together to tell your brain where your head is. They tell your brain if your head is straight up-and-down or sideways. This helps your brain tell your muscles how to move to keep your balance.

Problems with the inner ear can cause dizziness. Motion sickness, such as seasickness or carsickness, comes from your inner ear. When big waves shake you up, the liquid inside your inner ear sloshes around. That can make you feel sick.

WHAT CAUSES HEARING PROBLEMS?

Sometimes people are born with hearing problems. Injuries and infections can also cause hearing problems. People with hearing problems hear less well than others. People with very serious problems cannot hear any sounds. They are totally deaf (unable to hear).

Hearing problems can come from injured eardrums. Infections or other diseases can damage parts of the ear. Cancer can damage nerves. Children can be born with a missing or damaged cochlea or other ear parts. Some people slowly lose their hearing as they get older. Loud music and other loud noises can also cause hearing loss.

Doctors treat ear infections with medicines or use tubes to drain the ear. Ear doctors can give some people hearing aids, small machines that make sounds louder. They can give people with very serious hearing loss a cochlear implant. The implant is a tiny artificial cochlea that changes sound waves to signals. The signals are fed to the nerves that lead to the brain.

Hiccups
 

[B][SIZE="5"]HICCUPS[/SIZE][/B]

How do you get rid of hiccups? Do you hold your breath? Do you breathe into a paper bag? Maybe you have someone scare you? Or do you just wait until they go away?

Everyone has a favorite cure for hiccups. But why we get them is a mystery to science. They seem to serve no purpose.

WHAT ARE HICCUPS?

Hiccups start from the diaphragm (pronounced DY-uh-fram). This is a wide muscle below your lungs. It’s a very important muscle because it helps you breathe. It pulls down to help draw air into your lungs. It relaxes and moves up to help you breathe out.

Normally, this happens smoothly. But sometimes, the diaphragm pulls down with sudden jerk, called a spasm. This causes you to breathe in quickly. When this happens, the opening into your windpipe shuts. The flow of air is cut off. That’s what makes the hic sound.

FACTS ABOUT HICCUPS

Usually, people have several hiccups in a row. After a few minutes, they usually stop. All mammals (animals that raise their young on milk) get hiccups. Newborn human babies have them. In fact, even unborn babies can get hiccups! Women and men get hiccups equally as often. But pregnant women rarely get hiccups.

ARE HICCUPS DANGEROUS?

Almost all hiccups last a short time and are harmless. But in those rare cases when they last for several days or more, they interfere with sleeping and eating. If hiccups last more than a month, they must be treated with drugs or surgery. The longest recorded attack of hiccups is 60 years!

Nose and Smell
 

[B][SIZE="5"]Nose and Smell[/SIZE][/B]

Here’s a question: If you lost your nose, would you still be able to smell? Believe it or not, the answer is yes! Even though the nose is the organ of smell, it’s only the outside part. There’s an inside part that you can’t see. That’s where the smelling gets done.

THE NOSE

There are all kinds of noses in the animal world: big or small, flat or round, and long or short. In people, what we call the nose is the formation of bone, cartilage (tough tissue), and skin on the front of the face. The nose has two openings called nostrils. They allow air to come in and go out. When that happens, the nose collects the molecules of substances that cause odors.

The nose has other functions, too. The stiff hairs in the nostrils help keep out dust, dirt, and insects that you might take in when you breathe. Once inside the nose, the air you breathe is warmed and moistened in the big nasal cavity before going to the lungs. The nasal cavity is the big space behind your nostrils. It’s what gets stuffed up and swollen when you have a cold.

The size, shape, and health of your nose help determine the way your voice sounds. Pinch your nose shut and talk. Can you hear the difference?

SMELL

Smell is the detection of odors. It’s one of the five senses. The others are touch, taste, sight, and hearing.

Smelling takes place deep inside the nasal cavity. That’s because there are nerve endings called olfactory nerves located there. You actually smell stuff somewhere roughly between your eyes!

When a molecule that represents an odor hits the olfactory nerves, these nerve endings send a signal to your brain. The brain then determines what the smell is, and you recognize it. Most people can detect about 10,000 different odors! Unfortunately, one of them is dirty socks.

Smell plays a big part in the sense of taste, too. The taste buds can only detect four tastes: sweet, sour, salty, and bitter. Your sense of smell adds to these tastes. That’s why food is so rich and varied in taste. That’s also why if your nose is blocked up, your taste buds don’t work well either.

ANIMAL NOSES

For most animals, the nose and sense of smell are crucial to survival. They help animals find food. They can help animals tell friends from enemies and find mates.

Many animals have noses with extra talents. The elephant’s trunk is long like an arm and useful like a hand. It’s a trumpet when an elephant calls, and it’s a hose that allows an elephant to drink or give itself a shower.

Horses and camels can open and close their nostrils the way you can open and close your eyes. It helps them keep dirt or sand out when the wind is blowing.

A young salmon will travel thousands of miles downriver and into the ocean to live. Years later, it can use its sense of smell to return to the exact place it was born. Its ability to smell is that good! Can you imagine finding your way home from school only using your nose?

Skin
 

[B][SIZE="5"]SKIN[/SIZE][/B]

Your skin is your body’s largest organ, larger than your heart or lungs or brain. Skin covers your whole body, from the top of your head to the soles of your feet. You could not live without your skin. Skin keeps germs and other harmful things out of your body. It keeps water, blood, and other fluids that you need inside your body.

You are always losing your skin and growing new skin. The outside layer of your skin is dead. It flakes off all the time. New skin grows to replace it.

LAYERS OF SKIN

Your skin has an outside layer called the epidermis. Under that is a layer called the dermis. Under those two layers is a layer of fat.

The epidermis protects the layers beneath it from minor scrapes, jabs, and sunburn. The bottom of your epidermis is always making new skin cells. The dead cells on the top of your skin are constantly dropping off.

The dermis is full of nerves and blood vessels. The nerves send signals to your brain when you touch something. The dermis gives your skin its toughness and strength. It also helps keep your body cool in hot weather and warm in cold weather. A scrape only bleeds if it’s deep enough to go through the epidermis and into the dermis.

There is a layer of fat under the dermis. This layer is thicker than both top layers of skin.

WHAT GIVES SKIN ITS COLOR?

The top layer of skin has a chemical called melanin. Melanin gives your skin its color. The more melanin you have, the darker your skin is. Patches of skin with extra melanin make freckles.

Melanin protects your skin from the Sun’s harmful rays. Your skin makes more melanin when you are out in the Sun. More melanin gives you a tan.

Melanin has its limits. Too much sunlight is bad for your skin. Harmful rays from the Sun can burn your skin. They can cause skin cancer.

WHAT IS HAIR MADE OF?

Hair is made of skin cells. A hair grows from a root under the skin. The hair grows up inside a shaft through the layers of skin. The part of the hair that you see is made of dead cells.

The color of hair, like the color of skin, comes from melanin. Melanin is made in the hair root. The hair of older people turns gray because the root stops making melanin.

The shape of the hair shaft gives you straight or curly hair. A round shaft makes straight hair. An oval-shaped shaft makes wavy hair. A comma-shaped shaft makes curly hair.

WHAT ARE FINGERNAILS MADE OF?

Fingernails and toenails are also made of skin cells. These nail cells are very hard. Nail cells start growing inside your fingers and toes. The cells die. New cells push the dead cells outward. This makes nails on your fingers and toes grow.

WHAT CAUSES PIMPLES?

Plugged pores cause pimples (zits). Pores are openings in the skin. Dead skin cells can plug up a pore. The plug can make a blackhead or a whitehead on top of the skin.

Pores allow oil to get to the surface of your skin. This oil is the kind that gives you oily skin, not the kind that gets made into gasoline. The oil keeps your skin and hair from drying out. Groups of cells called glands make the oil. Oil glands are almost everywhere in your body. There are many oil glands on your face and scalp.

The oil cannot get out of a plugged pore. Soon a pimple or a red lump forms. The red lumps can leave scars. Squeezing a pimple can cause a scar.

Doctors can help teenagers with lots of pimples. Doctors can give them medicine that will help make the pimples go away.

WHAT MAKES A SCAB?

Have you ever scraped your knee or cut your finger? Scabs grow over scrapes, cuts, and other wounds that bleed. The blood dries and hardens. The blood turns into a scab.

New cells grow under the scab. The cells join up and cover the cut. When the skin is healed, the scab drops off

Teeth
 

[B][SIZE="5"]TEETH[/SIZE][/B]

They can bite, chomp, chew, crush, nibble, and gnaw. They can be cleaned, crowned, drilled, filled, pulled, capped, and straightened. You can crack them, break them, brush them, and floss them. You can even replace them if you lose them. What are they?

They’re your teeth.

WHAT TEETH ARE

Teeth are the hardest parts of the human body. They often survive long after the bones, or skeleton, have decayed. Scientists have discovered ancient teeth from animals and humans. These remains help scientists learn about the past. For example, they can tell whether a dinosaur ate other animals or plants from the kind of teeth it had.

Teeth are hard because they are made mainly of dentin and covered in a thin layer of enamel. Dentin is like bone only harder. Enamel is the hardest material in the body. Within this hard structure is a soft pulp full of blood vessels and nerves. If you have a toothache, it’s probably because something is irritating a nerve in the pulp.

Yet hard as they are, teeth can decay. Have you ever had a cavity? A cavity is a hole in a tooth. It develops when tiny bacteria eat away at the enamel and dentin. Luckily, a dentist can fill the holes that bacteria make.

Once a tooth decays, it can more easily crack and break. That’s why it’s so important to keep teeth clean. Brushing your teeth helps get rid of bacteria and the bits of food that bacteria feed on.

WHAT TEETH DO

Teeth do lots of things. They help you eat by tearing, grinding, and chewing food. It’s step one in the process of digestion.

Teeth also help you talk. Say the word thistle. Did you feel your tongue touch your upper front teeth? You should have felt it twice, on the th and on the l. You need your teeth to create certain sounds in speech.

Teeth help determine how you look. They support muscles in your face. If you didn’t have teeth, your lips would collapse inward.

KINDS OF TEETH

Teeth are specialists. That’s why your teeth don’t all look alike. If your adult teeth have grown in, you’ve got four different kinds of teeth.

Your front teeth, or incisors, are flat and sharp. You use these teeth like knives to cut into food. Next to the front teeth are your canines (your fangs!). You use these sharp, pointy teeth for tearing and shredding food. Beyond the canines are the bicuspids and the back teeth, or molars. You use these flatter teeth to chomp and grind.

ANIMAL TEETH

Animals have even more specialized teeth. They use their teeth for more than just eating.

Beavers use their front teeth to gnaw down whole trees. A beaver’s front teeth keep on growing. They would lengthen by about 4 feet (1.2 meters) every year if the beaver didn’t chisel them down by gnawing.

Walruses use their huge canines as hooks when they climb up onto ice. Elephant tusks are the largest teeth in the world. Elephants use them for digging or as weapons.

Piranhas are fish that have scissor-like teeth. They use these teeth to cut flesh off prey. Sharks have rows and rows of razor-sharp teeth. If they lose a tooth, a new one grows quickly in its place. The narwhal is a whale that has only two teeth. In male narwhals, one tooth grows forward like a long, twisted sword. Scientists are unsure about the purpose of these teeth, but they have seen narwhals dueling with them.

Some poisonous snakes have fangs for teeth. They use their fangs like needles for injections. These snakes bite and deliver deadly poison through their fangs!

Disease
 

[B][SIZE="5"]Disease[/SIZE][/B]

“Is it catching?” People sometimes ask this question when they hear that someone is sick. They are talking about a kind of human disease. The kinds of diseases that are contagious (catching) are caused by microscopic germs. Diseases caused by germs are called infectious diseases. Other human diseases can be caused by many things.

WHAT DISEASES DO GERMS CAUSE?

Pneumonia, strep throat, and food poisoning are some of the diseases caused by germs called bacteria. Bacteria are living things made of just one cell.

Germs called viruses cause colds, chicken pox, flu, mumps, measles, polio, and AIDS. Viruses are even tinier than bacteria.

A germ called a fungus causes athlete’s foot. Malaria is caused by a germ called a protozoan. Protozoans are made of one cell and are like tiny animals. Mad cow disease may be caused by a strange bit of protein called a prion.

HOW DO GERMS SPREAD?

Many germs spread in coughs and sneezes. Sick people get these germs on their hands. You can pick up these germs by touching a doorknob or something else that a sick person touched. Washing your hands regularly can help keep you safe from many diseases caused by germs.

Long ago, many people in Europe and North America died from diseases called cholera and typhoid. The germs that cause these diseases live in dirty water. Good sewers and clean water stopped the spread of these diseases in many countries. These diseases are still a problem in poor countries.

WHAT ELSE CAUSES DISEASE?

Contact with poisons can cause disease. Smoking tobacco can cause lung cancer and other lung diseases. Drinking lots of alcohol can damage the brain and liver. Lead in drinking water or paint can cause mental problems in children.

Clogged blood vessels cause heart disease. Many older people get diseases caused by body parts wearing out. Arthritis, Parkinson disease, and Alzheimer’s disease are diseases that most often occur as people age.

Sometimes, babies are born with diseases or birth defects. Sometimes babies can inherit (get from their parents) a disease that occurs later in life. Diabetes, high blood pressure, and muscular dystrophy are diseases that people can inherit.

Some diseases are caused by the body’s disease-fighting system. This system is called the immune system. Allergies come from the immune system fighting too much. Touching poison ivy can cause an itchy rash. The rash is a sign of an allergy.

Asthma is a breathing problem caused by the immune system. People can die from a bad asthma attack.

Not eating the right foods can cause disease. Not having enough vitamins can cause some diseases. A lack of vitamin C causes scurvy. Scurvy makes the skin bleed and teeth fall out. Hundreds of years ago, sailors on long voyages often died of scurvy.

WHAT ARE THE SIGNS OF DISEASE?

Chills and fever are often signs of diseases that are caused by germs. Pain can be a sign of disease. A diseased appendix or other organ causes pain. Itchy eyes and a runny nose can be a sign of hay fever or other allergy. Trouble breathing can be a sign of asthma.

Some diseases have no signs that you can see or feel. You cannot see or feel high blood pressure. Some diseases can only be found during a physical exam by a doctor. It is important to have regular checkups.

HOW DO DOCTORS TREAT DISEASES?

Doctors treat many kinds of diseases with medicines. Doctors can kill most disease-causing bacteria with drugs called antibiotics. Doctors give drugs to lower high blood pressure.

Doctors cannot really cure diseases caused by viruses. Your body can fight off colds and flu. The body cannot fight off AIDS, polio, and some other diseases caused by viruses. Doctors have drugs that can slow the AIDS virus. Doctors can vaccinate you against some diseases caused by viruses. Vaccinations help your body fight off disease. There are vaccines against chicken pox, flu, measles, polio, and some other viruses.

Doctors treat some diseases with surgery. They use surgery to take out diseased organs or tumors.

Some diseases doctors cannot yet cure or even treat. Researchers in laboratories are looking for ways to help people with these diseases.

Doctors tell people it is best to keep from getting diseases. Getting plenty of sleep and exercise and eating the right foods can help keep you healthy.

Cancer
 

[B][SIZE="5"]CANCER[/SIZE][/B]

Cancer—it’s scary word, and a scary disease. Cancer kills a lot of people all over the world. Only heart disease kills more Americans.

But there’s good news too. Millions of people who have had cancer are still alive. Doctors have learned a great deal about treating and preventing cancer.

WHAT IS CANCER?

Cancer is not a single disease. It includes more than 100 different diseases. They may affect any part of the body. But they have one thing in common. They are all caused by cells that are out of control.

All living things are made up of cells. An adult human body has about 30 trillion cells—that’s 30,000,000,000,000! Cells reproduce (make more cells) by dividing in half. In an adult body, about 25 million cells divide every second. That’s how the body heals itself.

Sometimes a cell goes out of control and divides over and over. And that’s what cancer is—unhealthy cells, growing and reproducing out of control. These cells are said to be cancerous.

WHY CANCER IS DANGEROUS

When cancerous cells multiply, they form clumps called tumors. Tumors can interfere with important body processes. Cancer of the lungs, for instance, interferes with breathing. Cancer of the stomach interferes with digesting food.

Cancerous cells can also spread to other parts of the body. Then new tumors form. This spreading is called metastasis. Cancer that has metastasized is the most dangerous. When cancer attacks several parts of the body, it is hard to stop.

HOW DOCTORS TREAT CANCER

The best weapon against cancer is detecting it at an early stage before it grows very much. Regular checkups by a doctor can detect cancer before it grows and spreads. People whose cancers are discovered early usually survive.

Some cancerous tumors can be removed by surgery. Doctors must remove some surrounding healthy cells, too, to be sure they get all the cancerous cells.

Radiation, such as X rays, can also kill cancer cells. So can treatment with powerful drugs, called chemotherapy. Unfortunately these treatments destroy healthy cells too. They can make people very sick. Newer treatments encourage the body’s own disease-fighting immune system to destroy cancer cells. But the newer treatments don’t work for all types of cancer.

WAYS TO PREVENT CANCER

There is no sure way to avoid cancer. But there are things people can do to greatly reduce the risk of getting cancer.

Smoking causes cancer. People who smoke get lung cancer 20 times more often than people who don’t smoke. Don’t smoke!

Doctors suspect that eating certain foods can also increase the chances of getting cancer. Eating lots of red meat and other foods high in saturated fat may make people more likely to develop cancer.

Most skin cancer is caused by too much Sun. Avoid getting sunburned. If everyone wore sunscreen or stayed out of the Sun, most cases of skin cancer would be prevented.

Colds and Flu
 

[B][SIZE="5"]Colds and Flu[/SIZE][/B]

You start sneezing. Your throat gets sore and scratchy. You have to blow your nose a lot. You don’t feel very well. Do you have cold? Or do you have the flu?

Colds and flu are illnesses caused by germs. They are both caused by germs called viruses. But they are caused by different kinds of viruses. Colds are often called common colds. Flu is short for influenza.

IS IT A COLD OR THE FLU?

Signs that you have a cold or the flu are called symptoms. The symptoms of a cold and the symptoms of the flu are slightly different.

The symptoms of a cold are sore throat, cough, sneezing, and a stuffy, runny nose. Colds usually do not cause a fever.

Flu symptoms are like cold symptoms, but the flu also causes chills, fever, and headaches. It makes you feel tired and achy all over.

A cold or the flu usually lasts about a week. Every once in a while they can lead to a more serious sickness, such as an ear infection or a lung infection called pneumonia.

CATCHING A COLD OR THE FLU

People once thought you could catch a cold from getting a chill in cold weather. They thought that wet feet or drafts of cold air could give you a cold.

We now know that germs cause colds and flu. The germs are passed from one person to another. They travel in coughs and sneezes.

More colds and cases of flu happen in cold weather because people spend more time together indoors when it’s cold outside. It is easier for germs to spread when people are close together.

IS THERE A CURE?

There is no cure for the common cold. People take medicine to help their sore throats, coughs, and runny noses. Doctors say that resting in bed is the best way to treat a cold.

There is no cure for the flu, either. Doctors can give medicine to make you feel better. Resting in bed and drinking lots of juice and water is the best way to treat the flu.

AVOIDING A COLD OR THE FLU

You can get a flu shot to help keep you from catching the flu. A flu shot, or vaccination, helps your body fight off flu germs if they attack. But it doesn’t always work. The flu virus keeps changing. When it changes, the old vaccine no longer works. Doctors have to keep making new vaccines. For this reason, you need a new flu shot every year. Sometimes the flu changes enough in a single year that you can still catch it even if you’ve been vaccinated.

There is no vaccine against the common cold because more than 100 different kinds of viruses cause colds. These viruses also keep changing.

One thing you can do to protect against cold and flu germs is wash your hands before you eat anything or touch your face. Your hands may pick up the germs from door knobs or other things touched by someone with a cold. Washing your hands thoroughly kills the germs.

You can also try not to spread germs when you have a cold or the flu. Use tissues when you sneeze. Cover your mouth when you cough. And wash your hands frequently to keep from spreading cold germs to others.
.

Chicken Pox
 

[B][SIZE="5"]Chicken Pox[/SIZE][/B]


Boys and girls sometimes wake up with a fever, headache, and sick feeling. Then, little red bumps pop up on their faces, scalps, and bodies. Everyone knows what this means: chicken pox!

Soon, the little bumps turn into horribly itchy blisters. If the kids scratch the blisters, the blisters can get infected. An infected blister could leave a scar. Next, the blisters break open and form scabs. The scabs fall off in about two weeks. Chicken pox is no fun.

WHAT CAUSES CHICKEN POX?

A kind of germ called a virus causes chicken pox. Viruses are so tiny that you can only see them under special, powerful microscopes. There are no drugs that can cure chicken pox. Antibiotics do not kill chicken pox viruses. Antibiotics do not work on any kind of virus.

Doctors tell kids with chicken pox to stay in bed. Warm baths and lotions are good for helping the itching. Kids with chicken pox should not take aspirin. Aspirin can cause another disease called Reye’s Syndrome in kids with chicken pox.

COULD I GET CHICKEN POX?

You can get vaccinated against chicken pox. Vaccines get your body ready to fight off germs. You probably will not get chicken pox if you have been vaccinated against the virus. The vaccine is usually given to babies when they are about one year old, so you may have already gotten the shot!

The chicken pox vaccine helps your body learn what the chicken pox virus is like. When the chicken pox virus comes around after you’ve been vaccinated, your body will kill the virus right away. You will not get sick. The body’s ability to resist a disease is called immunity. Doctors are not sure how long the immunity will last. You might need to get a booster shot one day.

IS CHICKEN POX CATCHING?

Chicken pox is a contagious disease. If you haven’t been vaccinated, you can catch chicken pox from someone who has the disease. Chicken pox germs spread in coughs and sneezes. You can also catch chicken pox from touching someone’s chicken pox blister.

You would not know right away that you had chicken pox. The germs can live in a body for two weeks before causing sickness. This time is called the incubation period.

People with chicken pox can give it to someone else, starting about two days after they begin to feel sick. Chicken pox is “catching” until scabs form on all the blisters. It takes about a week for scabs to form. Kids sick with chicken pox are kept away from others during this time. They do not go to school.

CAN GROWN-UPS CATCH CHICKEN POX?

Grown-ups can catch chicken pox. They can catch the disease if they did not have it when they were children and if they were not vaccinated. Grown-ups with chicken pox get much sicker than kids do.

Grown-ups who had chicken pox as kids cannot catch it again. Their bodies recognize the chicken pox virus and kill it right away. They have lifetime immunity from chicken pox.

Measles
 

[B][SIZE="5"]MEASLES[/SIZE][/B]

“Ooh, that light is too bright! What are these red dots all over my skin?”

A kid wakes up one morning with the measles. It started out like a cold, with sneezing and a runny nose. Then along came a fever. Finally red dots spread from head to toe. The rash of red dots is the most familiar sign of measles. Measles can also make eyes sensitive to light.

Almost all kids used to get measles. Then doctors invented a measles vaccine. Now a couple of shots is all it takes to prevent measles. In the year 2000, only about 100 kids in the United States caught the disease.

IS MEASLES CATCHING?

You can catch measles from someone who has the disease. A kind of germ called a virus causes measles. Coughing and sneezing spreads the virus from one person to another.

You probably will not catch measles if you have already had measles before. Your body learns to fight off the measles virus once the virus has attacked. You also probably will not catch measles if you have had a measles vaccination.

If you do catch measles, there is not much you can do. Kids who catch measles have to stay in bed. They have to stay away from other people so that they do not spread the germs. They can put lotion on their rash, but doctors still have no way to kill the measles virus. Drugs called antibiotics don’t work on viruses.

HOW SERIOUS IS MEASLES?

People with measles usually get better in about two weeks. Sometimes measles can make a body very weak. Other germs can attack. These germs can cause lung and ear infections.

Sometimes measles harms the brain. The virus can get into the brain and cause a disease called encephalitis. People can die from encephalitis, but this does not happen very often.

IS MEASLES THE SAME AS GERMAN MEASLES?

Measles and German measles are two different diseases. They are caused by different viruses. German measles has some of the same signs as measles. But German measles does not make people as sick or last as long. German measles usually attacks older kids and young adults.

You probably were vaccinated against German measles. There has been a German measles vaccine since the 1970s. Now, only a few hundred people a year in the United States get German measles.

HAS MEASLES BEEN WIPED OUT?

Measles is now rare in Canada, the United States, and most European countries. Kids in Africa, Asia, and other parts of the world still catch measles. Poor countries cannot afford to vaccinate all the children. The World Health Organization (WHO) says that about 1 million children in poor countries die from measles each year. Scientists are working on cheaper vaccines.