The Systems of the Body

Click to see Vol. I, Issue 2 of the newsletter in which this article first appeared.

Listen to the audio of this article! (6:28)

A one-room hut needs little in the way of electrical, plumbing, and other systems, but a large mansion will need much more.

The same is true of living organisms. A single-celled animal doesn't need any "systems." But we are much more complex, so it takes many systems to keep our body operating.

People count the systems in different ways (sometimes combining systems, sometimes separating them). Let's take a look at one way of thinking about them.

--------

The Skeletal System: These are the bones that make up the framework that keeps the body standing up. Altogether we have around 270 bones when we are born; as we grow up some of these join together, and an adult has around 206. The bones also have other functions, such as producing red blood cells in the bones' center, called marrow.

The Muscular System: There are three types of muscles:

• Skeletal muscles move the bones.
• Smooth muscles are "involuntary"--we don't control them, as we do the skeletal muscles. They line some of the body's "tubes," like blood vessels and intestines. They are also found in the eye, the skin, and elsewhere.
• Cardiac muscle makes up the heart.

The Nervous System: The body's nerves collect and process information from the senses, and tell the skeletal muscles what to do. The system is made up of linked-together neurons ("nerve cells").

The structure of the nervous system has two main parts:

• the central nervous system (CNS) is the brain and spinal cord; and
• the peripheral nervous system (PNS) branches off of the CNS into other parts of the body.

The nervous system functions in two ways:

• the somatic (meaning "of the body") nervous system is connected to the skin, joints, and muscles, and operates mostly voluntary actions, like moving your hand.
• the autonomic nervous system connects to organs, blood vessels, glands, and the like, and controls functions like breathing and digestion. These actions are involuntary.

The sensory organs--eye, ear, nose, mouth, and skin--are also part of the nervous system.

The Endocrine System: This system influences the function of the body through the production and release of "chemical messengers" called hormones. They are produced in organs called endocrine glands--such as the pineal gland, pituitary gland, pancreas, thyroid gland, and adrenal glands--which secrete the hormones directly into the blood.

The Circulatory (or Cardiovascular) System: This circulates blood around the body, delivering oxygen and nutrients to the body's cells and carrying away their waste products. The heart pumps the blood through arteries away from the heart, into the capillaries (where transfer happens), and the veins back to the heart.

• Blood is made up of plasma (a liquid), red blood cells (carrying oxygen), white blood cells (part of the immune system), and platelets (which cause the blood to coagulate or clot).

The Lymphatic System: This system defends the body against harmful "invasions." Some consider it part of the circulatory system. Lymph vessels drain impurities out of the body, and carry lymphocytes ("white blood cells") to the site of infection. There are also about 450 (in adults) centers called lymph nodes.

The Immune System: This system is better thought of as a network of biological processes than as a system of structures, like the skeletal or other systems. To say someone has a "strong immune system" is simply to say that her body is good at fighting off infection.

The Respiratory System: This system brings oxygen into the lungs, from which it is absorbed into the bloodstream and carried out to the parts of the body by the circulatory system. That system then returns "spent" blood to the lungs, where its carbon dioxide is removed and breathed out again.

• Parts include the mouth and nose, pharynx and larynx, trachea, and (inside the lungs) bronchi and alveolae. The diaphragm, a muscle, keeps the whole system moving.

The Digestive and Excretory System(s): This system brings food into the body, digests it into nutrients (with the influence of enzymes and hormones), and expels the waste products.

• Its parts include the gastrointestinal tract: the mouth, esophagus, stomach, and intestines. The salivary glands, pancreas, liver, gallbladder, and other organs are also important to digestion.

The Urinary and Renal System(s): This is where the kidneys filter impurities out of the blood and produce urine, which is then carried through the ureters to the bladder, where it is stored until carried out of the body through the urethra.

The Reproductive Systems: These are made up of the organs required for the production of children. They include:

• in males: the testes (which produce sperm), seminal vesicles, prostate, vas deferens, urethra, and penis
• in females: the ovaries (which produce ova or "eggs"), fallopian tubes, uterus, cervix, and the vagina and vulva

The Integumentary and Exocrine System(s): This is the body's "exterior": the skin, hair, and nails, as well as exocrine glands that secrete sweat, skin oils, saliva, mucous, tears, and earwax.

---

Activities

Vocabulary Exercise 1

Match the words to their meaning. Answers in the first comment.

1. the Circulatory System
2. the Digestive and Excretory System(s)
3. the Endocrine System
4. the Immune System
5. the Integumentary and Exocrine System(s)
6. the Lymphatic System
7. the Muscular System
8. the Nervous System
9. the Reproductive Systems
10. the Respiratory System
11. the Skeletal System
12. the Urinary and Renal System(s)

1. brings in oxygen and removes carbon dioxide
2. brings in nutrients and expels waste
3. collects and processes information
4. covers the body's outside
5. drains impurities from the body
6. fights off infection
7. makes up the body's framework
8. make babies
9. makes things move in the body
10. moves the blood
11. produces and removes urine
12. produces hormones

Vocabulary Exercise 2

Using the numbered systems above, match the parts to their system. Answers in the first comment below.

1. ovaries and testes
2. glands and hormones
3. kidneys and bladder
4. vessels and nodes
5. white blood cells
6. central and autonomic
7. heart, arteries, and veins
8. lungs and diaphragm
9. skin and sweat glands
10. mouth, pancreas, and stomach
11. skeletal, smooth, and cardiac
12. bones