CorpusHuman Anatomy

Blood Vessel Wall

Artery, vein and capillary compared — layers, lumen and flow

Top: cross-sections of a muscular artery, an accompanying vein cut through one of its valves, and a capillary. The capillary is drawn about 300 times larger than it really is. Arteries and veins share the same three layers, or tunics.

The tunica intima is the endothelium on a thin subendothelial layer, bounded in arteries by a wavy internal elastic lamina. The tunica media is circular smooth muscle, ending at an external elastic lamina. The tunica externa (adventitia) is collagen carrying the vessels' own small blood vessels (vasa vasorum). The artery has a thick, muscular media and a small, round lumen. The vein has a thin media, a relatively thick adventitia and a wide, irregular lumen that holds most of the blood volume. A capillary is only a single layer of endothelium on a basement membrane, with an occasional pericyte, and a red cell (≈7.5 µm) only just fits through it.

Bottom: flow in each vessel. In the artery, pressure and flow surge with every heartbeat and the elastic wall bulges, which is why you can feel a pulse. In the vein, flow is slow and steady at low pressure, and valve cusps point toward the heart. In the capillary, red cells pass in single file, deforming as they go.

Step-by-step

  1. 1 · Tunica intima

    The innermost layer is a single layer of endothelium, a thin subendothelial connective tissue layer and, in arteries, a fenestrated internal elastic lamina. The endothelium controls vascular tone (NO, endothelin), clotting and leukocyte traffic. Atherosclerosis is a disease of the intima.

  2. 2 · Tunica media

    The middle layer is made of circular smooth muscle with elastic and collagen fibres, bounded by the external elastic lamina. It is thickest in arteries, where its contraction and relaxation set vessel diameter and resistance (Poiseuille: R ∝ 1/r⁴). In veins it is thin.

  3. 3 · Tunica externa (adventitia)

    The outer coat is mostly type I collagen. It anchors the vessel and carries the vasa vasorum and nerves. It is usually the thickest layer of a vein, and large veins also have longitudinal smooth muscle in it.

  4. 4 · Artery versus vein

    An artery has a thick media and a small, round lumen. It works at high, pulsatile pressure and its elastic wall expands with each beat. A vein has a thin wall and a large, irregular lumen. It works at low, steady pressure, holds most of the blood (capacitance) and has valves, folds of intima that stop backflow.

  5. 5 · Capillary

    The capillary wall is a single endothelial cell layer on a basement membrane, partly wrapped by pericytes. The lumen (≈5–10 µm) lets red cells through only in single file. Flow is slow (≈0.3 mm/s) and the wall is thin, which is ideal for diffusion and Starling exchange.

Labelled parts