Anatomy and Physiology II Practice Test

Capillaries are the type of blood vessels responsible for the exchange of substances between blood and tissues. These tiny vessels are crucial because their structure allows for maximum permeability; they are composed of a single layer of endothelial cells, which facilitates the easy passage of oxygen, carbon dioxide, nutrients, and waste products between the blood and surrounding tissues. The extensive network of capillaries provides a large surface area for these exchanges to occur efficiently, making them essential for maintaining cellular function and homeostasis.

In contrast, arteries and veins function primarily to transport blood. Arteries carry blood away from the heart under high pressure, while veins return blood to the heart under lower pressure. Although veins and venules do participate in transport and can contain some exchange functions, their primary role is not the direct exchange of nutrients and waste as seen in capillaries. Hence, the unique structural characteristics of capillaries make them distinctly suited for the critical function of substance exchange.