The Circulatory System

The human circulatory system can be called the internal mass transit system. This is because the circulatory system is the means by which substances in the body move from one location to another. Food, oxygen, heat, and wastes are all transported by the circulatory system.

After food is digested in the stomach, it travels down the small intestine. Millions of capillaries surround the tissues of the small intestine. As the food molecules travel down the intestine, they are actively or passively diffused into the capillaries and the blood stream. In there, these molecules move to all of the body’s other tissues where they will be used to grow, maintain, and reproduce new cells.

Oxygen is obtained in the lungs, where there are microscopic air sacs called alveoli. These alveoli are surrounded by capillaries, which are low in oxygen. As the air rushes into the alveoli, passive diffusion moves the oxygen into the capillaries where it is picked up by the hemoglobin in the erythrocytes and transported to other tissues in the body. At the same time, carbon dioxide, a waste product from cellular respiration, in the blood stream diffuses into the alveoli to be exhaled.

Heat is also a vital resource that is transported in the blood stream. As cells produce and use energy, heat is produced. This heat is vital for mammalian cells to function at such a high metabolic rate. Heat produced in the internal organs warms the blood. As the blood moves from the heart and to the other tissues, the cells are kept warm. When the blood returns to the heart, it reheated and starts the cycle over again.

Another important function of the circulatory system is the transport of wastes. As cells go through their normal metabolic functions, wastes such as carbon dioxide and urea are produced. The transport and removal of carbon dioxide has been discussed above. Urea, a nitrogenous waste, is produced by the cells and if it is not removed, then it will begin to kill the cells. As the blood moves past cells, urea is moved into the blood stream. An important artery, the renal artery, transports blood into the kidneys where it will be ‘filtered’ to remove the waste. The urea is then transported by the ureters, structures of the urinary system, to the bladder to be excreted.