Blood Vessel Structure

Blood Vessel Structure

An artery’s wall consists of three distinct layers and blood-containing space known as the lumen. The innermost tunica intima is made up of a layer of simple­ squamous epithelium known as the endothelium­ (1). It rests on a connective tissuemembrane with many elastic, collagenous fibers. The endothelium helps prevent blood clotting and may also help in regulating blood flow. It releases nitric oxide to relax smooth muscle of the vessel. In arter-ies, the outer margin has a thick layer of elastic fibers known as the internal elastic membrane.

The middle tunica media (2) makes up most of an arterial wall, including smooth muscle fibers arranged mostly in circles and a thick elastic con-nective tissue layer. The smooth muscle fiber activity is controlled by many chemicals and the autonomic nervous system’s sympathetic vasomotor nerve fibers. The tunica media is separated from the next layer, the tunica externa, by a thin band of fibers known as the external elastic membrane.

The outer tunica externa (3), also known as the tunica adventitia, is thinner, made mostly of connective tissue with irregular fibers. It is attached to the sur-rounding tissues (FIGURE 19-2) and contains many lymphatic vessels and nerve fibers. The tunica externa of larger blood vessels contains many tiny blood vessels,­ which comprise a system known as the vasavasorum. This system nourishes the outer tissuesof blood vessel walls. The luminal inner portions of ­vessels obtain nutrients directly from the blood.

In arteries and arterioles, vasomotor fibers receive impulses to contract and reduce blood vessel­ diameter, a process known as vasoconstriction. When inhibited, the muscle fibers relax and the vessel’s­ diameter increases in a process known as ­vasodilation. Changes in artery and arteriole diame-ters greatly affect blood flow and pressure.