Microvessels irregate the tissues and gas and nutrient exchanges occur in the venules and capillaries. Arterioles are smaller and have thicker walls than venules. Only the tunica media and intima remain. In venules, the smooth muscle cells of the media are arranged longitudinally with respect to the vessel wall and in arterioles, they are circularily disposed. In smaller venules the media is not present and only a layer of endothelial cells remain. Capillaries are the smallest of vessels and often only one erythrocyte can fit within the lumen. In these cases only one endothelial cell wraps around to join with itself. Capillaries are often invested with pericytes which encircle the vessles with attentuated processes.