Which type of blood vessel is the smallest?

Capillaries are the smallest type of blood vessel in the circulatory system. They are tiny, thin-walled vessels that connect arterioles (small arteries) and venules (small veins). Their primary function is to facilitate the exchange of oxygen, carbon dioxide, nutrients, and waste products between the blood and the surrounding tissues.

Capillaries have a diameter of approximately 5 to 10 micrometers, which is just large enough for red blood cells to pass through in single file. This small size enhances their surface area relative to volume, thereby maximizing the efficiency of exchange during the process of diffusion. The thin walls of capillaries are composed of a single layer of endothelial cells, allowing for easy movement of substances between the blood and tissues.

In contrast, arteries and veins are larger and are designed to transport blood over longer distances within the body. Arterioles, while smaller than arteries and larger than capillaries, serve as resistance vessels to regulate blood flow and are still significantly larger than capillaries. Thus, recognizing the unique characteristics and roles of each vessel helps clarify why capillaries are classified as the smallest blood vessels.