Which type of blood vessel has no tunica media or adventitia?

Capillaries are the smallest type of blood vessels in the circulatory system, characterized by their single layer of endothelial cells that allows for the exchange of nutrients, gases, and waste products between blood and surrounding tissues. Unlike larger blood vessels such as arteries and veins, capillaries do not have a tunica media, which is the layer composed of smooth muscle and elastic tissue, nor do they possess an adventitia, which is the outer connective tissue layer. The lack of these layers is essential for their primary function, which is to facilitate efficient diffusion due to their minimal barrier.

The structure of capillaries enhances their ability to transport substances effectively, as the thin walls allow for easier movement of materials. This physiological adaptation is a key reason why capillaries are vital in all forms of transport throughout the body, facilitating processes such as oxygen delivery to tissues and the removal of carbon dioxide and other metabolic wastes.