What term is used for the vessels that play a key role in the exchange of materials between blood and tissues?

Capillaries are the small blood vessels that serve as the primary sites for the exchange of materials, such as oxygen, carbon dioxide, nutrients, and waste products, between the blood and the tissues of the body. Their walls are extraordinarily thin, consisting of just a single layer of endothelial cells, which facilitates this exchange process. This thinness allows for easier diffusion of substances due to the minimal distance between the blood and the surrounding tissue cells.

In contrast, arteries are responsible for carrying oxygenated blood away from the heart to the various tissues of the body, while veins carry deoxygenated blood back toward the heart. Veinules are small veins that collect blood from capillaries and transport it towards larger veins, but they do not play a direct role in the exchange of materials. Thus, the unique structure and function of capillaries make them essential for the critical exchange processes necessary for maintaining homeostasis within the body.