What are capillaries primarily responsible for?

Capillaries play a crucial role in the circulatory system primarily through their function of exchanging nutrients and waste materials. These tiny blood vessels, which have walls that are one cell thick, facilitate the transfer of oxygen, carbon dioxide, nutrients, and waste products between the blood and the surrounding tissues. This process occurs through diffusion, allowing essential substances to reach cells and enabling waste products to be removed effectively.

The structure of capillaries makes them perfectly suited for this function, as their extensive surface area and thin walls maximize contact with tissue cells. This is essential for maintaining proper cellular function and overall homeostasis within the body. In contrast, the other options relate to different aspects of the circulatory system, but they do not capture the primary function of capillaries.