What type of blood vessels are capillaries?

Capillaries are thin-walled blood vessels that facilitate the exchange of gases, nutrients, and waste products between the blood and surrounding tissues. Their structure is specifically adapted for this function, featuring walls that are only one cell thick. This thinness allows for efficient diffusion, enabling oxygen and nutrients to move from the blood into cells, while carbon dioxide and metabolic wastes can be transported from the cells into the blood.

In contrast to other types of blood vessels, capillaries lack the muscular walls found in arteries and veins, as their primary role is not to transport blood over long distances but to partake in the crucial process of nutrient and gas exchange. The other choices reflect attributes not associated with capillaries, such as muscular characteristics (which pertain to arteries), the presence of valves (which are found in veins), and the limitation of only carrying oxygenated blood (a function of arteries and not capillaries, which transport both oxygenated and deoxygenated blood). This focus on the structural and functional role of capillaries underlines why they are described specifically as thin and allowing for gas and nutrient exchange.