Figure 1

Blood vessel composition. Large blood (arteries and veins) vessels are composed of three layers (shown here out of proportion). Tunica intima (the thinnest layer, facing the blood) contains a single layer of endothelial cells positioned on subendothelial extracellular matrix and circularly arranged elastic bands called the internal elastic lamina. Tunica media (which is the thickest layer in arteries) is composed of extracellular matrix, smooth muscle cells (SMCs) and thick elastic band called external elastic lamina. Tunica adventitia (which is the thickest layer in veins) is made of connective tissue with interspersed fibroblasts and stem/progenitor cells. It also contains nerve endings and nutrient capillaries (vasa vasorum) in the larger blood vessels.