The network of capillaries in the body is extensive, allowing capillaries close contact with cells of the body. Very few living cells lie farther than 0.01 mm from a capillary. Why do cells need to be close to capillaries? The close proximity of capillaries, along with their thin walls, allows for the exchange of materials between the blood and the body’s cells. As blood passes through a capillary, oxygen, glucose, and other substances in high concentration move out of the capillary toward nearby cells. Then those substances move across cell membranes and into the cells. At the same time, carbon dioxide and other waste products that are in high concentration inside the cells move across membranes into the blood carried by the capillary. The circulatory system is just one example of a system of exchange in the human body. Right now, your circulatory system is making thousands of exchanges a second. At the same time, your urinary system and gas exchange system are making many thousands more exchanges. It makes working in a high-rise building seem simple, doesn’t it?

1. What are the main components of the circulatory system?

2. What is the function of the gas, circulatory, and urinary system?

3. How do each of these systems return to homeostasis?

4. Why do you think it is important for these systems to maintain homeostasis?