What are the three main functions of the circulatory system?

by Jtigger on October 25th, 2007

The human circulatory system has three main parts: (1) the heart, (2) the blood vessels, and (3) the blood. A watery fluid called lymph, and the vessels that carry it, are sometimes considered a part of the circulatory system .
The circulatory system performs many vital functions. It plays an important role in respiration. The circulatory system is also important in nutrition, in the removal of wastes and poisons, and in several other body processes.
In respiration. The circulatory system plays a part in respiration by delivering oxygen to the cells and removing carbon dioxide from them. During this process, the blood follows two routes called the systemic circulation and the pulmonary circulation.
From the left side of the heart, blood full of oxygen is pumped into the systemic circulation. This blood leaves the heart through the aorta, the main artery of the body. A number of major arteries branch off the aorta. These arteries, in turn, branch into smaller and smaller vessels, finally emptying into the tiny capillaries. There, oxygen leaves the blood and enters the tissues through the thin capillary walls. In a similar way, carbon dioxide leaves the tissues and enters the blood. The blood, now carrying carbon dioxide, leaves the capillaries and flows through larger and larger veins. Eventually, the blood enters the right side of the heart through two large veins--the superior vena cava, which carries blood from the head and arms, and the inferior vena cava, which carries blood from the trunk and legs.
From the right side of the heart, the blood is pumped into the pulmonary circulation. Pulmonary arteries carry the blood that contains carbon dioxide to capillaries in the lungs. The carbon dioxide passes through the capillary walls into the lungs and is then exhaled. Oxygen that has been inhaled passes from the lungs into the blood in a similar way. The blood returns through the pulmonary veins to the left side of the heart and begins its journey again.
In nutrition. The circulatory system carries digested food substances to the cells of the body. These nutrients enter the bloodstream by passing through the walls of the small intestine into the capillaries. The blood then carries most of the nutrients to the liver.
The liver removes certain nutrients from the blood and stores them. It later releases the nutrients into the blood when the body needs them. The liver also changes some nutrients into substances needed by the body. Blood leaving the liver contains nutrients that the cells use in the production of energy, enzymes, and new building materials for the body.
In removal of wastes and poisons. The circulatory system helps dispose of waste products and poisons that would harm the body if they accumulated. These substances include carbon dioxide, salts, and ammonia, a by-product of the cell's use of protein.
The liver removes ammonia and other wastes, and various poisons that enter the body through the digestive system. The liver converts the wastes and poisons into water-soluble substances, which are carried by the blood to the kidneys. The kidneys filter out these and other water-soluble wastes and expel them from the body in urine.
Other functions. The circulatory system helps protect the body from disease. White blood cells engulf and destroy bacteria, viruses, and other harmful invaders. As the blood circulates, it also helps keep the body temperature stable by absorbing heat from the cells' production of energy.
If the temperature of the body begins to rise, the flow of blood into vessels in the skin increases. The heat from cells deep in the body is thus carried to the skin, from where it passes out of the body. If the temperature of the body begins to drop, the flow of blood to the skin is restricted. This action enables the body to retain as much heat as possible.
The circulatory system also carries hormones. These chemical substances affect or control the activities of various organs and tissues. Hormones are produced by the endocrine glands, including the thyroid, pituitary, adrenal, and sex glands. These glands release their hormones into the bloodstream.

http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookcircSYS.html

http://www.kidshealth.org/parent/general/body_basics/heart.html

http://library.thinkquest.org/28807/data/circ2.htm

http://www.heartzine.com/anatomy-physiology/193-Functions+of+the+Circulatory+System.html