What do capillaries primarily facilitate?

Capillaries primarily facilitate gas exchange and substance diffusion due to their unique structure and function. They are the smallest blood vessels in the body, creating a vast network that penetrates almost all tissues. The walls of capillaries are thin and permeable, consisting of just a single layer of endothelial cells. This thinness allows for efficient exchange of oxygen, carbon dioxide, nutrients, and waste products between the blood and surrounding tissues.

In areas where gas exchange occurs, such as in the lungs and tissues, oxygen diffuses from the capillaries into the cells, while carbon dioxide moves from the cells into the capillaries to be transported away. Additionally, nutrients and hormones can diffuse into tissues through capillaries, further supporting cellular function and metabolism.

The other options, while related to biological processes, do not primarily involve capillaries. Blood clotting is a process primarily involving platelets and clotting factors, hormone distribution is conducted through larger blood vessels and systemic circulation, and electrical signaling is related to nerve cells rather than the function of capillaries. Thus, the correct answer highlights the capillaries' crucial role in sustaining life through effective material exchange.