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.
