FIGURE 13.11 • Schematic diagram of the major structural features of blood vessels. The layers or tunics of the blood vessel walls are labeled in the upper two panels. The arrangement of the microcirculatory bed in certain parts of the body is shown in the lowest panel. Note the location of pericytes and their relationship to the basal lamina. Also, an arteriovenous (AV) anastomosis is shown within the microcirculatory bed.
 The tunica intima, the innermost layer of the vessel, consists of three components: (a) a single layer of squamous epithelial cells, the endothelium; (b) the basal lamina of the endothelial cells (a thin extracellular layer composed chiefly of collagen, proteoglycans, and glycoproteins); and (c) the subendothelial layer, consisting of loose connective tissue. Occasional smooth muscle cells are found in the loose connective tissue. The subendothelial layer of the intima in arteries and arterioles contains a sheetlike layer or lamella of fenestrated elastic material called the internal elastic membrane. Fenestrations enable substances to diffuse readily through the layer and reach cells deep within the wall of the vessel.
 The tunica media, or middle layer, consists primarily of circumferentially arranged layers of smooth muscle cells. In arteries, this layer is relatively thick and extends from the internal elastic membrane to the external elastic membrane. The external elastic membrane is a layer of elastin that separates the tunica media from the tunica adventitia. Variable amounts of elastin, reticular fibers, and proteoglycans are interposed between the smooth muscle cells of the tunica media. The sheets or lamellae of elastin are fenestrated and arranged in circular concentric layers. All of the extracellular components of the tunica media are produced by the smooth muscle cells.
 The tunica adventitia, or outermost connective tissue layer, is composed primarily of longitudinally arranged collagenous tissue and a few elastic fibers. These connective tissue elements gradually merge with the loose connective tissue surrounding the vessels. The tunica adventitia ranges from relatively thin in most of the arterial system to quite thick in the venules and veins, where it is the major component of the vessel wall. In addition, the tunica adventitia of large arteries and veins contains a system of vessels called the vasa vasorum that supplies blood to the vascular walls themselves, as well as a network of autonomic nerves called nervi vascularis that control contraction of the smooth muscle in the vessel walls.
Histologically, the various types of arteries and veins are distinguished from each other by the thickness of the vascular wall and differences in the composition of the layers. Table 13.1 summarizes the features of the various types of blood vessels.
In the adult human body, a circulatory system consists of about 60,000 miles of different-sized vessels that are lined by a simple squamous epithelium called endothelium. The endothelium is formed by a continuous layer of flattened, elongated, and polygonally shaped endothelial cells that are aligned with their long axes in the direction of the blood flow. At the luminal surface, they express a variety of surface adhesion molecules and receptors (i.e., low-density lipoprotein [LDL], insulin, and histamine receptors). Endothelial cells play an important role in blood homeostasis. The functional properties of these cells change in response to various stimuli. This process, known as endothelial activation, is also responsible for the pathogenesis of many vascular diseases (e.g., atherosclerosis; Folder 13.1). Inducers of endothelial activation include bacterial and viral antigens, cytotoxins, complement products, lipid products, and hypoxia. Activated endothelial cells exhibit new surface adhesion molecules and produce different classes of cytokines, lymphokines, growth factors, and vasoconstrictor and vasodilator molecules, as well as molecules that control blood coagulation.