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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 breath, heal wounds, digest food, process sounds, 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.

Check out the major systems of your body below.

Circulatory system

The circulatory 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, while blood vessels deliver the blood to the rest of the body. The blood carries oxygen and nutrients to cells throughout the body. One path has blood  moving from the heart to the lungs and back to the heart again. The other path moves blood from the heart to the rest of the body and back to the heart again.

There are different types of blood vessels:

  • Arteries carry blood away from the heart
  • Veins return blood to the heart
  • Capillaries are very thin 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 206 bones, giving the body its shape and support. Bones also protect the brain, spine, organs such as the heart and lungs, and the reproductive organs.

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 called stem cells. Stem cells 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 to fight infection

The muscoloskeletal 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 also muscle (called cardiac muscle), pumping out blood as it beats.

Nervous system

The central nervous 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, emotions, and addictions too. The brain also takes in information in the form of signals from the 5 senses: sight, hearing, touch, smell, and taste.

Information is sent to and from the brain using nerves, which connect to the spinal cord and extend to other parts of the body. These 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 a person to cover his or her 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, movements on the left side of the body are controlled by the right side of the brain. The left side of the brain controls the right side of the body.

Digestive system

The digestive system includes a group of hollow organs joined as a long, twisting tube from the mouth to the rectum, where waste leaves the body.

The tube is lined with what is called the mucosa. In the mouth, stomach, and small intestine, the mucosa has little glands (special groups of cells) that make juices that help to digest food. Two solid organs, the liver and pancreas, also make juices that help with digestion. The nerves and blood, parts of other systems, help with this process, as well. This is 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 in order to give fuel to the rest of the body.

The digestive process begins in the mouth, continues down the esophagus, to the stomach, small intestines, and the large intestines. The hollow organs of the digestive system have muscles that help to 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 system delivers oxygen, which is fuel or energy, 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, which 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 also does other jobs, such as protect against things that cause infections. Using tiny hair called cilia (say: sih-lee-uh), the respiratory system works to clean the air breathed in by sweeping things like dust out of the body.

Endocrine system

The endocrine system is in charge of the chemicals that the body makes to control growth, metabolism (how the body makes and uses energy), sexual development, and reproductive function. These chemicals are called hormones, which are released into the blood and taken to tissues and organs throughout the body. The endocrine system makes sure that there is a normal amount of the hormones in the blood — not too much and not too little.

Most of the body’s hormones are made inside of special groups of cells called glands. Among the major glands that make up the endocrine system, some control the function of other glands, while some glands have their own special jobs. Some of these jobs include making hormones that affect growth, the feeling of pain, appetite, body temperature, and reproduction. For example, the adrenal glands help respond to stress, and the ovaries are involved in breast development and other processes.

Reproductive system

During puberty, changes in estrogen and other hormones spark the start of the menstrual cycle, which includes having a period and the hormonal changes that take place over about one month.

The ovaries — which are two small glands (special groups of cells) — release or let go of one egg about once a month, from the 1 million or so eggs it has been storing since before birth (called ovulation). The egg moves along a fallopian tube, which connects the ovary to the uterus (or womb), a hollow organ where a baby grows. This takes several days. During this time, the lining of the uterus becomes thicker to make itself a better home for a baby. A woman will get pregnant if she has sex without birth control (such as a condom) with a male, and his sperm fertilizes or 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.

The vagina, which is made of muscle, is a hollow tube that can grow wider to deliver a baby that has finished growing inside the uterus. The cervix is the narrow entryway in between the vagina and uterus.

The breasts are also an important part of the reproductive system. When women become pregnant, their breasts produce 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 is fed by sucking milk from the mother’s nipple.

Learn more about your reproductive health.


Content last reviewed October 13, 2010
Page last updated October 31, 2013

U.S. Department of Health and Human Services, Office on Women's Health.