Final answer:
Blood vessels, excluding capillaries, have three layers: the tunica intima, tunica media, and tunica externa. Capillaries have only a single layer, the tunica intima, which facilitates the exchange of substances with tissues.
Step-by-step explanation:
Blood vessels, with the exception of capillaries, are composed of three distinct layers, which are often referred to as tunics. These layers are:
- Tunica intima: This is the innermost layer, which has direct contact with the blood flow inside the vessel. It is comprised of a smooth lining of endothelial cells.
- Tunica media: This middle layer is thicker in arteries and contains smooth muscle and elastic fibers that help control the diameter and flow of the blood vessels.
- Tunica externa (or adventitia): It is the outermost layer, which is primarily composed of connective tissue and serves to protect and provide structure to the vessel.
In contrast, capillaries consist only of a single layer of endothelial cells, the tunica intima, which allows for the exchange of substances between the blood and surrounding tissues.