Blood Vessel Structure
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: Figure 2 shows the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and the tissues; and the veins, which carry blood from the capillaries back toward the heart.
The arteries and veins are both composed of three layers. Tunica intima (the thinnest layer) is the innermost layer. It is a single layer of simple squamous endothelial cells glued by a polysaccharide intercellular matrix. It is surrounded by a thin layer of subendothelial connective tissue interlaced with a number of circularly arranged elastic bands called the internal elastic lamina. Tunica media (the thickest layer) is circularly-arranged elastic fiber, connective tissue, and polysaccharide substances. This layer is thinner in the veins than in arteries, as veins collapse when not filled with blood and do not function primarily in a contractile manner. The second and third layer is separated by another thick elastic band called external elastic lamina.
The tunica media (especially in arteries) may be rich in vascular smooth muscle, which controls the caliber of the vessel. Tunica externa or adventitia, the outermost layer, is entirely made of connective tissue. It also contains nerves that supply the vessel, as well as nutrient capillaries (vasa vasorum) in the larger blood vessels. Veins often display a large number of anatomical variations compared with arteries within a species and between species. Veins are often closer to the skin and contain valves the help keep blood flowing towards the heart (Figure 3).
A capillary consists of little more than a layer of endothelium and occasional connective tissue. When blood vessels connect to form a region of diffuse vascular supply it is called an anastomosis (pl. anastomoses). Anastomoses provide critical alternative routes for blood to flow in case of blockages.