This layer contains collagen fibers, elastic fibers, and fibroblasts. In larger examples of muscular arteries an ex ternal elastic lamina lays adjacent to the tunica media. The tunica adventitia is usually about the same thickness as the tunica media. 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 .