For normal blood vessels, both arterial and venous, they are generally divided into three layers, mainly the intima, the mid-vessel and the epithelium. The intima is mainly composed of endothelium, subendothelium and internal elastic membrane. The endothelium is mainly a single layer of flat epithelium, which has a smooth surface and facilitates the flow of blood. The intima of blood vessels is mainly composed of elastic membrane, smooth muscle and elastic fibers, as well as a small amount of building tissue. The outer membrane of blood vessels is mainly composed of loose connective tissue, and some blood vessels have an outer elastic membrane.