Final answer:
The function of capillaries is to serve as thin-walled vessels that facilitate the exchange of nutrients and gases between the blood and the body's tissues. They consist of a single layer of endothelial cells, allowing for efficient diffusion and transport of various substances.
Step-by-step explanation:
The function of capillaries is a) Thin-walled vessels; site of nutrient and gas exchange. Capillaries are the smallest of blood vessels and consist of a single layer of endothelial cells. This layer is crucial for the process of perfusion, which is the supply of blood to the body's tissues. The diameter of a capillary lumen ranges from 5-10 micrometers, allowing an erythrocyte, or red blood cell, to pass through one at a time. The wall of the capillary is where the exchange of oxygen, carbon dioxide, nutrients, and wastes occurs between the blood and the surrounding cells and tissue fluid. This exchange is facilitated by the thinness of the capillary walls, which allows for diffusion, facilitated diffusion, and other methods such as endocytosis and exocytosis to transport various substances in and out of the bloodstream.