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Old Thursday, September 10, 2009
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Default Human Body

Human Body

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.
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Default Cell

CELL

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.
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Default Genes and Heredity

Genes and Heredity

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.
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Default Bones and Skeleton

Bones and Skeleton

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.
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Default Brain and Nervous System

Brain and Nervous System

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.
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Default Heart and Circulation

Heart and Circulation

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.
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Default Lungs and Breathing

Lungs and Breathing

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.
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Default Digestive System

DIGESTIVE SYSTEM

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.
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Default Muscles

MUSCLES

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
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Default Eye

EYE

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!
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