Unit 4 · How the Body Works

Put your hand on your chest. Can you feel hard ridges? Those are your ribs. Ribs are part of your skeleton. The skeleton is the frame of bones inside your body. An adult has 206 bones.

Babies are born with more bones than adults. Many of their bones are soft at first. As a child grows, some bones join together. That is why a grown-up ends up with 206. The longest bone is the thigh bone, called the femur.

Bones do many jobs. They hold you up, like the frame of a house. They also protect soft parts. Your skull guards your brain. Your ribs guard your heart and lungs. Inside many bones is soft marrow, which makes new blood cells.

Bend your arm and make a fist. Feel the top of your arm get firm. That is your biceps muscle at work. A muscle is body tissue that can squeeze and pull. You have more than 600 muscles.

Muscles can only pull. They cannot push. So many muscles work in pairs. The biceps pulls to bend your arm. The triceps, on the back of your arm, pulls to straighten it. Strong cords called tendons tie muscles to bones.

Some muscles work without you thinking. Your heart is a muscle that never takes a break. Muscles in your stomach squeeze food along. The muscles you choose to move, like the ones in your legs, help you walk, write and play.

Try to pick up a pencil without bending your fingers. It is very hard! Your fingers bend at joints. A joint is a place where two bones meet. You have joints in your knees, elbows, hips and neck.

Joints move in different ways. Your elbow and knee work like a door hinge. They bend back and forth. Your shoulder and hip are ball-and-socket joints. A round end of one bone fits into a cup in another. So your arm can swing in a circle.

The ends of bones at a joint are covered with cartilage. Cartilage is smooth and a little bendy. You can feel it in the tip of your nose and your ears. It helps bones slide past each other without rubbing.

Make a fist. Your heart is about the size of your fist. It sits in your chest, a little to the left. The heart is a muscle that pumps blood. Each squeeze is one heartbeat.

For hundreds of years, many people thought blood was made in the liver and used up. In 1628, an English doctor named William Harvey tested this. He measured and watched. He showed that the same blood goes around and around the body. This loop is called circulation.

Your heart beats about 100,000 times a day. When you rest, it goes slower. When you run, it speeds up. You can feel the beat in your wrist or neck. That beat is called your pulse.

Think of a city with big highways and tiny side streets. Your body has roads for blood. They are called blood vessels. Together, the heart and blood vessels make up the circulatory system.

Arteries carry blood away from the heart. This blood is full of oxygen, a gas your cells need. Veins carry blood back to the heart. You can sometimes see blue lines on your wrist. Those are veins near the skin.

Between arteries and veins are capillaries. Capillaries are so thin that blood cells go through them in single file. There, blood drops off oxygen and food for the cells. It picks up waste to carry away.

Take a slow, deep breath. Feel your chest rise. Air is filling your lungs. You have two lungs in your chest. They are soft and spongy. Together with your nose, mouth and windpipe, they form the respiratory system.

A strong, flat muscle sits under your lungs. It is called the diaphragm. When it moves down, air rushes in. When it moves up, air goes out. You breathe many times a minute without thinking.

Inside the lungs, air reaches tiny sacs. Oxygen passes from the sacs into the blood. At the same time, a waste gas called carbon dioxide leaves the blood. You breathe it out. This trade happens with every breath.

You take a bite of an apple. Your teeth chop it. Your spit, called saliva, makes it soft. Then you swallow. The food slides down a tube called the esophagus. This is the start of digestion, the way the body breaks down food.

Next the food lands in your stomach. The stomach is a stretchy bag of muscle. It squeezes and mixes food with strong juices. After a few hours, the food is a thick soup.

The soup moves into the small intestine. This long, folded tube takes the useful parts into the blood. Then the large intestine takes back water. What the body cannot use leaves as waste.

You touch a hot pan and pull your hand back fast. How did that happen so quickly? Your nerves did it. Nerves are thin threads that carry messages through your body. They are part of the nervous system.

The brain is the control center. It sits inside your skull. It helps you think, remember, feel and move. It also runs things you never notice. It keeps you breathing, even while you sleep. It speeds up or slows down your heartbeat.

A thick bundle of nerves runs down your back. It is called the spinal cord. Your backbone protects it. Messages zoom up the spinal cord to the brain. Answers zoom back down to your muscles, very fast.

Think back to Maya’s race. Her leg muscles needed more energy. To make energy, muscles need oxygen. So her whole body changed at once. Each group of parts that does a job is called a body system.

Her lungs breathed faster to take in more oxygen. Her heart beat faster to carry that oxygen to her legs. Her brain sent messages to keep all of this going. No system worked alone.

When the race ended, her body slowly returned to rest. Keeping things steady inside the body is called balance. Your body is always working to keep that balance, whether you are running or sleeping.

Look at the back of your hand. The skin you see is an organ. An organ is a body part that does a special job. Skin is the largest organ of all. On an adult it covers about 20 square feet.

Skin is a shield. It keeps germs and dirt out. It keeps water in. It has nerves that let you feel a soft kitten, a cold ice cube or a sharp pin. It also tells you when something hurts.

Skin also helps control your temperature. When you get hot, you sweat. Sweat is salty water that comes out of tiny holes. As it dries, it cools you down. That is what Maya’s skin did in the race.

Food gives your body things it needs. These are called nutrients. There are six main kinds: carbohydrates, protein, fats, vitamins, minerals and water. Each one has its own job.

Carbohydrates are your body’s main fuel. They are in rice, bread, pasta, tortillas, fruit, beans and potatoes. Your body turns them into a sugar that your cells burn for energy. Your brain uses a lot of it.

Some grains are whole grains, like brown rice, oatmeal and whole-wheat bread. They keep the outer part of the grain. That part has fiber. Fiber helps food move through your intestines and helps you feel full longer.

A scraped knee heals in a week or two. New skin grows over the cut. To build that skin, your body uses protein. Protein is a nutrient that builds and repairs body parts.

Protein comes from many foods. Fish, chicken, eggs, milk and meat have protein. So do plant foods like beans, lentils, tofu, peanuts and seeds. Rice and beans together are a protein-rich meal eaten in many countries.

Your muscles, skin, hair and nails are built with protein. Growing children need protein every day. Eating it from different foods is a good way to get enough.

Some people think all fat is bad. That is not true. Your body needs some fat. Fat stores energy. It helps your body use some vitamins. It also helps build your brain and nerves.

Fats come in different kinds. Olive oil, nuts, seeds, avocados and fish have fats that are good for the heart. These are often called healthy fats. Many cooks use them every day.

Butter, cheese and fatty meats have another kind of fat. It is fine in small amounts. Fried snacks and many sweets have a lot of fat and sugar. Doctors say to enjoy them sometimes, not every day.

Long ago, sailors on long trips got very sick. Their gums bled and they felt weak. In the 1700s, a Scottish doctor named James Lind tested different foods. The sailors who ate oranges and lemons got better. Today we know those fruits have vitamin C.

Vitamins are nutrients your body needs in small amounts. Vitamin C helps heal cuts. Vitamin A, in carrots and sweet potatoes, helps your eyes. Vitamin D helps your bones. Your skin makes some vitamin D in sunlight.

Minerals come from the earth and get into plants and animals. Calcium, in milk, yogurt, tofu and leafy greens, builds strong bones and teeth. Iron, in beans, spinach and meat, helps blood carry oxygen. Eating many colors of fruits and vegetables helps you get many vitamins and minerals.

After gym class, your mouth feels dry. You head to the drinking fountain. Your body is asking for water. Water is one of the six nutrients. You can live much longer without food than without water.

Much of your body is water. It is in your blood, your muscles and even your bones. Water carries nutrients to your cells. It helps you digest food. It helps make sweat to cool you down.

You lose water all day when you breathe, sweat and use the bathroom. So you need to drink more. Plain water and milk are good choices. Fruits like watermelon and oranges have water too. Feeling thirsty is a sign your body needs water; being low on water is called being dehydrated.

Picture a dinner plate cut into four parts, with a small cup beside it. This picture is called MyPlate. The U.S. Department of Agriculture made it in 2011 to help families plan meals.

On MyPlate, fruits and vegetables fill half the plate. Grains fill about a quarter. Protein foods fill the last quarter. The cup stands for dairy, like milk or yogurt. Soy milk with added calcium counts too.

MyPlate is a guide, not a strict rule. It works with foods from any culture. A bowl of rice with fish and greens fits. So do beans, tortillas and salsa. The idea is balance: some of each group over the day.

Pick up a box of cereal and turn it around. You will find a white box with black letters. It is called the Nutrition Facts label. Most packaged foods in the United States must have one.

Start at the top. Find the serving size. This is the amount the numbers are about, like one cup. If you eat two cups, you get twice as much of everything. Next, find calories. Calories tell how much energy the food gives.

Then look down the list. You will see fat, sodium, carbohydrates, fiber, sugars and protein. The label also shows added sugars. Added sugars are sugars put in when the food is made. The right side shows a percent Daily Value. As a rough guide, 5 percent is low and 20 percent is high.

Your teeth are covered in enamel. Enamel is the hardest part of your body. But tiny germs live in every mouth. When you eat sugar, the germs eat it too. They make acid. Over time, acid can make holes in teeth called cavities.

Sticky candy and sugary drinks are hardest on teeth. Soda, sweet tea and juice wash sugar over teeth again and again. Water is a great drink for your teeth. The World Health Organization advises that people keep added sugars low.

You can help your teeth every day. Brush two times a day with fluoride toothpaste. Fluoride is a mineral that makes enamel stronger. See a dentist for checkups. A sweet treat now and then is fine, especially with a meal.

In Brazil and Mexico, many families eat rice and beans. The rice gives carbohydrates. The beans give protein and fiber. Eaten together, they make a filling, healthy meal.

In India, many families eat dal. Dal is a warm stew of lentils cooked with spices like turmeric and cumin. It is often served with rice or flatbread. In China, families often stir-fry vegetables in a hot pan with a little oil.

In Japan and many coastal places, people eat a lot of fish. Fish gives protein and healthy fats. Each culture has its own foods. Many of them fit MyPlate well. You can enjoy your family’s foods and try new ones too.

You have not eaten all night. In the morning, your body needs fuel. Breakfast breaks that overnight fast. Many students find a good breakfast helps them focus and feel steady in class.

Breakfast can look many ways. Oatmeal with fruit. Eggs and a tortilla. Rice with an egg on top. Yogurt and a banana. Many schools serve free or low-cost breakfast too. A mix of a grain, a protein and a fruit gives lasting energy.

No single food is ’bad.’ A piece of birthday cake at a party is fine. What matters is the whole day and the whole week. Eating many kinds of foods is called variety. If you have questions about food and your body, ask a parent, school nurse or doctor.