#VASCULAR VESSELS The arteries and veins have the same basic structure. There are three layers, from inside to outside while the capillaries have only one thick cell. Tunica intima (the thinnest layer) - a single layer of simple squamous endothelial cells glued by a polysaccharide intercellular matrix, 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) - circularly arranged elastic fiber, connective tissue, polysaccharide substances, the second and third layer are separated by another thick elastic band called external elastic lamina. The tunica media may (especially in arteries) be rich in vascular smooth muscle, which controls the caliber of the vessel. Tunica adventitia: entirely made of connective tissue. It also contains nerves that supply the muscular layer, as well as nutrient capillaries (vasa vasorum) in the larger blood vessels. Capillaries consist 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. Laid end to end, the blood vessels in an average human body will stretch approximately 62,000 miles (100,000 km)--2 times around the earth © 2008 chae young kim