What is the primary role of capillaries in the circulatory system?

The primary role of capillaries in the circulatory system is to facilitate the exchange of nutrients and gases between blood and tissues. Capillaries are the smallest blood vessels in the body, and their structure is uniquely suited for this function. They have thin walls made up of a single layer of endothelial cells, allowing for easy diffusion of oxygen, carbon dioxide, nutrients, and waste products between the blood and surrounding tissues.

This exchange process occurs because of the concentration gradients: oxygen and nutrients in the blood diffuse out of the capillaries into the tissues, while carbon dioxide and other metabolic waste products from the tissues diffuse into the blood. The vast network of capillaries ensures that every cell in the body is in close proximity to the blood supply, maximizing the efficiency of this exchange process.

In contrast, the other options describe different functions that are not associated with capillaries. Transporting blood quickly across long distances is primarily the role of arteries, while veins are responsible for carrying blood back to the heart and often have mechanisms to prevent backflow, such as valves. Storage of blood is not a function of capillaries either; instead, blood is stored in larger vessels like veins or specialized organs. Thus, the unique structure and function of cap