The vascular system is composed of a complex network of tubular vessels allowing blood to circulate around the body. It consists of arteries (carrying oxygenated blood from heart), veins (carrying the deoxygenated blood from the tissues to the heart) and capillaries (the site of gas, nutrient and metabolite exchange) with veins and arteries located side by side in tissues.

Although veins and arteries have different functions, they share a number of anatomical features. The walls of the majority of vessels consist of three distinguishable layers: tunica interna (or intima), tunica media, and tunica externa (or adventitia).

The internal layer (tunica intima) is composed of the endothelial cell layer, constituting an interface with the blood. At their basal surface, the endothelial cells are connected to the underlying connective tissue. The endothelium in arteries and large veins is enveloped by a continuous layer of vascular smooth...