What role do capillaries serve in the circulatory system?

Capillaries are the smallest blood vessels in the circulatory system, and their primary role is to facilitate gas exchange between the blood and the tissues. Their thin walls, comprised of a single layer of endothelial cells, allow for the efficient diffusion of oxygen, carbon dioxide, nutrients, and waste products. This process occurs due to the differences in concentration gradients between the blood within the capillaries and the surrounding tissues.

Through this exchange, oxygen is delivered to cells, and carbon dioxide, a waste product of cellular respiration, is removed from them. This critical function ensures that tissues receive the necessary oxygen for metabolism and that waste is effectively transported for elimination from the body. The extensive network of capillaries in tissues maximizes this gas exchange process, providing a large surface area relative to volume.

While other options mention functions like transporting blood or regulating pressure, these tasks are primarily associated with larger blood vessels such as arteries and veins. Storing excess blood is not a function of capillaries, as they are not designed for storage but rather for facilitating immediate exchange and flow.