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Inside your body

Your body can do some amazing things. It can walk and chew gum at the same time — and clean the air you breathe, heal wounds, digest food, fight germs, and more. If you understand how it works, you can help keep your body from having problems and appreciate it when all goes well.

What are the major systems of your body? Check them out here.

Circulatory system

The circulatory (say: SUR-kyoo-luh-TOR-ee) system keeps blood moving throughout the body. It is made up of the heart, blood vessels, and blood.

The heart is a muscle that pumps out blood with each heartbeat. When the blood leaves the heart, it travels through blood vessels in two paths. One path takes blood from the heart to the lungs, where it picks up oxygen and then goes back to the heart. In the other path, blood goes from the heart to the rest of the body, where it delivers oxygen and nutrients to cells. Then the blood goes back to the heart and then out to the body all over again.

There are different types of blood vessels:

  • Arteries carry blood away from the heart.
  • Veins return blood to the heart.
  • Capillaries are very small blood vessels that connect the arteries and veins.

The heart has four areas called chambers:

  • The right and left ventricles, which pump blood out of the heart
  • The right and left atria, which collect the blood coming into the heart

Musculoskeletal system

The skeleton has bones, giving the body its shape and support. Bones also protect the brain, spine, and organs, such as the heart and lungs.

Bones also hold important minerals, such as calcium, which is what makes bones hard.

On the inside of bones, there is soft bone marrow. The bone marrow has special cells that make:

  • Red blood cells, which bring oxygen to the tissues in the body
  • Platelets, which help the blood clot (form a scab to stop bleeding)
  • White blood cells, which help fight infection

The musculoskeletal system includes not just the bones, but also tendons, joints, ligaments, cartilage, and muscles. These parts help people bend and move.

The body also has involuntary muscles that can't be controlled in the same way as the muscles in an arm or leg. These muscles make up the walls of the stomach and other body parts.

The heart is made of special muscle (called cardiac muscle) that pumps out blood as it beats.

Nervous system

The central nervous (say: NER-vuhs) system is made up of the brain and spinal cord, which runs down the back. The brain controls everything we do, from talking to moving, as well as basic body functions, such as digestion. The brain is involved in learning, remembering, and feelings, too. The brain also takes in information in the form of signals from the five senses: sight, hearing, touch, smell, and taste.

Information is sent to and from the brain using nerves. The nerves send signals, such as pain or noise, from other parts of the body to the spinal cord and then the brain. The brain then sends information back that tells the body how to respond. For example, a loud noise may cause the brain to tell you to cover your ears.

Different parts of the brain do different jobs. When it comes to movement, specific spots in the brain control certain body parts. In general, the right side of the brain controls the left side of the body, and the left side of the brain controls the right side.

Digestive system

The digestive (say: di-JES-tiv) system is basically a long, twisting tube that connects sacs (pouches). It goes from the mouth to the rectum.

The digestive process is necessary because food is not in a form that the body can use as fuel, or energy. Digestion breaks down food and drink into their smallest parts to give fuel to the rest of the body.

The digestive process begins in the mouth and continues down the esophagus to the stomach, small intestine, and large intestine.

In the mouth, stomach, and small intestine, little glands (special groups of cells) make juices that help to digest food. The liver and pancreas also make juices that help with digestion. The nerves and blood, which are parts of other systems, help with this process, too.

The digestive system has muscles that help move the food along. The food that is not completely digested leaves the body through the anus as a bowel movement.

Respiratory system

The respiratory (say: RES-puh-ruh-TOR-ee) system delivers oxygen, which serves as fuel, to cells throughout the body. This system includes:

  • Nose
  • Throat
  • Larynx (voice box)
  • Trachea (windpipe)
  • Lungs

Air is breathed in through the nasal passageway (nose) and the mouth, and travels through the trachea (windpipe) and bronchi. Bronchi are tubes that connect the windpipe to the lungs.

The lungs then move oxygen from the air into the blood so that it can reach the rest of the body. At the same time, the lungs remove carbon dioxide waste from the blood so that it can leave the body when a person exhales (breathes out).

The respiratory system has another job: protecting your body from things that cause infections. Using tiny hair called cilia (say: SIL-ee-uh), the respiratory system works to clean the air breathed in by sweeping things like dust out of the body.

Endocrine system

The endocrine (say: EN-doh-krun) system is in charge of hormones. Hormones are natural body chemicals that control growth, metabolism (how the body makes and uses energy), sexual development, and reproductive function. Hormones are released into the blood and taken to tissues and organs throughout the body.

The endocrine system makes sure that there is the right amount of each of the hormones in the blood — not too much and not too little.

Most hormones are made inside of special groups of cells called glands.

Some glands control other glands, while other glands have their own special jobs, including making hormones. For example, the adrenal glands make adrenaline, which helps you respond to stress. The ovaries make estrogen, which plays an important role in reproduction.

Reproductive system

During puberty, changes in estrogen and other hormones spark the start of the menstrual cycle.

The ovaries, which are two small glands (special groups of cells), can hold hundreds of thousands of eggs. The ovaries release one egg about once a month. This is called ovulation.

The egg moves along one of two fallopian tubes, which connect each ovary to the uterus (or womb), the organ where a baby grows. The egg moving through a fallopian tube takes several days. During this time, the lining of the uterus becomes thicker to make itself a better home for a baby.

Pregnancy happens when a sperm fertilizes (joins) the egg on its way to the uterus. A fertilized egg attaches itself to the lining of the uterus to begin growing into a baby. If the egg doesn't become fertilized, it will be shed along with the lining of the uterus during the next menstrual period.

When a baby is ready to be born, it moves out of the uterus and through the cervix, which gets bigger to make room for the baby and opens to the vagina. The vagina is a tube that widens to let the baby come out.

The breasts are also an important part of the reproductive system. After a woman gives birth, her breasts make milk to feed the baby. The milk is made in the cells of the lobules inside the breast and travels through ducts to the nipple. The baby sucks milk from the mother's nipple.

Learn more about your reproductive health.


Content last reviewed April 15, 2014
Page last updated June 13, 2014