A thin internal elastic lamina is inconsistently present. The adventitia is thin and poorly organized. Arterioles regulate the flow of blood to capillary beds. Neural stimulation of the precapillary sphincter reduces blood flow to a specific region. Thus blood is diverted to areas that require increased rates of blood flow.
Capillaries possess neither a tunica media nor a tunica adventitia.
Capillaries are classified as continuous, or fenestrated, or discontinuous according to their endothelium. Continuous capillaries . The endothelium forms a continuous layer around the circumference of the vascular lumen. The mechanism of transport across these vessels is under investigation. One model states that pinocytotic vesicles, commonly seen in the cytoplasm of endothelial cells, shuttle materials across the endothelium.