2. Endothelium and vasodilation

The vascular wall is composed of layers that can be identified by their respective morphology and by the different functions exhibited by respective cells which, ultimately, are responsible for the vascular tone, influencing blood pressure. Arteries and veins have a similar structure presenting three layers: intima or endothelium, media or smooth muscle and adventitia.

The tunica intima is the inner and thinnest layer and surrounds the lumen. It is made up of endothelial cells lining the entire vasculature and includes circular elastic bands, the internal elastic lamina. The tunica media, also called muscle layer, is composed of vascular smooth muscle, which helps regulate the size of the lumen and externally present circular elastic bands, the external elastic lamina. This tunica differs between arteries and veins: arteries contain more smooth muscle than the tunica media of their counterpart, the veins, and this allows arteries to constrict and dilate to adjust the volume of blood needed by the tissues that they support. Additionally, the structure of arteries differs between large arteries and resistant arteries: in the first type, arteries present a media with large amount of elastic fibers disposed between smooth muscle cells and the thickness of the vascular wall is thinner than that exhibited by resistant arteries that often have multiple strands of smooth muscle layers. The external layer, adventitia layer is composed of connective tissue allowing the blood vessel to withstand forces acting on the vessel wall and of collagen fibers that anchor the vessel to surrounding tissues.

The endothelium can evoke effects, dilation or contraction of the underlying vascular smooth muscle, by releasing endothelium-derived relaxing factors (EDRF) such as NO or endotheliumderived contracting factors (EDCF) such as endothelin or prostanoids.
