Human Body Systems
The 11 human body systems work together and make it possible to breathe, move, speak, digest food and carry out many other functions. Learn about each system to better understand how it works.
- One of the most important of the 11 human body systems is the nervous system. The nervous system consists of the central nervous system and peripheral nervous system. The job of the central nervous system is to receive information and send out instructions. This system consists of the brain and spinal cord. The peripheral nervous system consists of the nerves in the body. These nerves send out messages to other parts of the body. The brain acts as the control center of the body, as it helps control all of the organs and tissues. The brain also lets humans remember things and experience the five senses of touch, taste, smell, sight and hearing.
- The spinal cord consists of a thick bundle of nerves that connect the brain to the rest of the body. The vertebrae, small bones, protect the spinal cord from injury and damage. Four types of nerves help to control the body. Autonomic nerves link the brain and spinal cord to organs like the heart and intestines. Cranial nerves connect the mouth, ears, eyes and nose to the brain. Peripheral nerves link the spinal cord with the arms and legs. Central nerves connect structures within the spinal cord and brain.
- The skeletal system consists of all the bones and tissues that connect them together. This system protects the organs from injury, provides support for the body and allows the body to move. The skull protects the brain. The vertebral column protects the spinal cord. This column consists of the atlas and axis, cervical vertebrae, thoracic vertebrae, lumbar vertebrae and sacral vertebrae. Other common bones in the human body include the jawbone (mandible), breast bone (sternum), thigh bone (femur), ankle (tarsals) and kneecap (patella). The tissues that connect bones to muscle and bones to other bones include ligaments, tendons and cartilage.
- The muscular system consists of three types of muscle. Smooth muscle has several functions in the body. This type of muscle pushes food through the digestive tract, pushes food back up to the esophagus when someone vomits and helps push a baby out of the body during childbirth. Cardiac muscle is the muscle in the heart. This muscle relaxes and contracts to circulate blood throughout the body. Skeletal muscles aid in movement and support the body. Each major muscle group has different functions. The quadriceps, for example, help you stand up, walk and climb stairs.
- The circulatory system consists of the heart and blood vessels. These tissues circulate blood throughout the body. The heart consists of upper and lower chambers (atria and ventricles) and four valves that control the direction of blood flow. Arteries carry blood away from the heart, while veins carry blood toward the heart. Capillaries, the smallest blood vessels in the body, allow blood to circulate between the arteries and veins.
- The respiratory system controls breathing as a means of supplying the blood with oxygen. The blood then carries this oxygen to the organs and tissues of the body. During breathing, a person breathes in oxygen and releases carbon dioxide. The respiratory system consists of the lungs, diaphragm, mouth, trachea and nose. The mouth and nose allow oxygen to enter the body. The trachea carries the oxygen to the chest cavity, where it splits into bronchi. The bronchi split again and form bronchial tubes, which carry oxygen to the lungs. Oxygen passes into small sacs called alveoli and then diffuses into the arteries of the blood via the capillaries. Blood from the veins releases carbon dioxide into the alveoli and the carbon dioxide exits the body when a person exhales. The diaphragm is a sheet of muscles that help you inhale and exhale. This sheet of muscles also assists in the exhalation of carbon dioxide and inhalation of oxygen.
- The digestive system breaks down food and absorbs nutrients. Food enters the mouth, where it mixes with saliva. The saliva helps soften and break down the food so it is easier to swallow. From the mouth, food passes through the esophagus, a muscular tube that pushes the food into the stomach. The stomach contains enzymes and acids that break down food into smaller pieces. This makes the food easier to digest. Food moves from the stomach to the small intestine in the form of chyme, a thick liquid.
- The small intestine consists of the duodenum, jejunum and ileum. The small intestine also contains millions of villi, which are small projections that aid in the absorption of nutrients. Any undigested food in the small intestine moves to the large intestine. This organ removes water from the undigested food and forms feces, or solid waste. The rectum stores this solid waste until it is ready to leave the body.
- The endocrine system controls the production and secretion of hormones. Hormones control growth, sexual development, metabolism and other body functions. This system consists of the pituitary gland, hypothalamus, thyroid gland, pineal body, adrenal glands, parathyroid glands and the pancreas. The pituitary gland acts as the master gland because it produces hormones that play a role in the function of other endocrine system glands. This gland produces growth hormone, adrenocorticotropin, thyroid-stimulating hormone, luteinizing hormone, vasopressin, prolactin and oxytocin.
- The hypothalamus regulates metabolism, feelings of fullness after eating and body temperature. This gland also secretes hormones that control the release of hormones from the pituitary gland. The thyroid gland produces hormones that regulate metabolism. This gland also participates in growth and development of the nervous system, blood pressure regulation, digestion, reproduction, muscle tone and regulation of the heart rate. The pineal body secretes melatonin, which helps regulate the sleep cycle.
- The adrenal glands, located at the top of each kidney, consist of the adrenal medulla and adrenal cortex. The adrenal cortex produces corticosteroids to regulate fluid-salt balance, immune system function, metabolism and sexual function. The adrenal medulla produces hormones that control the body’s response to stress. The parathyroid glands control calcium levels in the blood and bones. The pancreas secretes digestive enzymes and secretes glucagon and insulin. Insulin and glucagon control the amount of glucose in the blood, so damage to the pancreas can lead to diabetes.
- The male reproductive system produces sperm, discharges sperm into the female reproductive system and produces male sex hormones that keep the reproductive system working normally. The external structures of this system include the penis, testicles and scrotum. The penis has a root that attaches it to the abdomen of the body. The urethra, which transports urine and semen, is at the tip of the penis. The scrotum contains the testicles, blood vessels and nerves. The scrotum controls the temperature of the testes, as they must remain at a temperature slightly lower than normal body temperature. This helps ensure normal development of the sperm. The testicles produce sperm and generate male sex hormones such as testosterone. Accessory organs of the male reproductive system include the vas deferens, epididymis, seminal vesicles and ejaculatory ducts.
- The female reproductive system has several functions. It produces eggs and transports eggs to the fertilization site. This system also participates in the gestation of a baby and the process of childbirth, menstruation and menopause. The external structures of the female reproductive system include the labia minora, labia majora, clitoris and Bartholin’s glands. The internal structures include the uterus, ovaries, vagina and fallopian tubes. The ovaries produce eggs and release them. The eggs travel down the fallopian tubes. The uterus plays a role in pregnancy and childbirth. The lining of the uterus, called the endometrium, builds up in preparation for fertilization. If fertilization does not occur, the uterus sheds its lining during menstruation. If fertilization does occur, the fertilized egg attaches to the wall of the uterus. The uterus holds the zygote, which becomes a fetus, throughout the 40 weeks of pregnancy. The vagina serves as the site of sexual intercourse and childbirth. The vagina expands to accommodate the fetus as it exits the body.
- The excretory system helps the body get rid of waste products. The major organs in this system are the kidneys, lungs and skin. Carbon dioxide and other waste gases exit the body via the lungs. The skin helps remove sweat and dead skin cells from the body. The kidneys remove wastes from the blood by filtering the blood, sending nutrients back into the body and expelling wastes in the urine. If kidney damage occurs, wastes could build up in the blood and cause a person to get very ill.
- The largest of the 11 human body systems is the Integumentary system. This system accounts for 12 to 15 percent of our body weight, according to Estrella Mountain Community College. This system consists of the skin, hair, nails and mucous membranes. This system controls body temperature, protects the body from damage, absorbs nutrients, helps maintain homeostasis and works with the nervous system to control the sense of touch. The Integumentary system also contains glands and hair follicles. Eccrine glands are found all over the body, while apocrine glands are found in the armpits and groin. The apocrine glands produce a substance that combines with bacteria to produce body odor.
- The immune system protects the body against harmful organism. This system consists of organs, tissues, proteins and specialized cells that help prevent infections and keep us healthy. White blood cells identify harmful organisms and destroy them. Produced by the bone marrow, thymus and spleen, leukocytes circulate in the blood vessels and lymphatic tissues. Phagocytes break up harmful organisms; lymphocytes help the body recognize organisms and destroy those organisms.
- Scientists classify lymphocytes as B lymphocytes and T lymphocytes. Lymphocytes that remain in the bone marrow are B lymphocytes. Other lymphocytes leave the bone marrow and travel to the thymus gland. These are T lymphocytes. B lymphocytes identify invaders and send out cells to destroy them. T lymphocytes destroy the invaders. Sometimes, the immune system mistakenly identifies normal tissues as foreign invaders. The system then attacks the normal tissues and causes tissue damage. This is known as an autoimmune disease. Autoimmune disorders include lupus, scleroderma and rheumatoid arthritis. A malfunctioning immune system also plays a role in allergic response.