The vascular system has a common histological organization that appears with particular clarity at the level of the large vessels. The tissue components (endothelium, smooth muscle, elastic elements and connective tissue) that form the vascular walls are arranged in concentric layers. Under the influence of local functional factors, this layered structure has undergone segmental differentiations that characterize each type of vessel. For descriptive purposes the concentric layers have been classified as three tunics as follows (from the lumen outward):

As a result of segmental differentiations, some features of these tunics are accentuated, others are reduced or omitted, or other additional structures are introduced. Accordingly, the detailed organization of the vascular wall varies characteristically from segment to segment.

Blood vessels are divided into two broad categories: macrovasculature and microvasculature. The macrovasculature is composed of those blood vessels that can be seen with the naked eye. The microvasculature is composed of blood vessels that are smaller than 100 microns may only be seen through the microscope. While the vessels of the macrovasculature appear as isolated anatomical entities, the microvasculature appears structurally and functionally as part of the tissue it supplies.

The sequential vascular segments can be classified as follows: elastic arteries (conducting arteries), muscular arteries (distributing arteries), arterioles, capillaries, venules (pericytic and muscular), and veins (small, medium, and large).

Elastic arteries are primarily represented by the large vessels emerging from the heart ventricles, such as the aorta and the pulmonary artery. Characteristic for these vessels are numerous concentric elastic lamellae of the tunica media interspersed with bundles of smooth muscle cells. Elastin can be stained with special stains. Elastic fibers in the media allow for the maintenance of blood pressure through the expansion and contraction of the vessel walls. The vasa vasorum, a network of small vessels that supplies the cells of larger vessels, is present in their adventitia and outer part of the media.

Muscular arteries follow the elastic arteries. Through the controlled contraction of their walls, these arteries distribute the blood to different parts of the body according to regional needs. The media of muscular arteries is composed of many concentric layers of smooth muscle cells arranged in a low angle helix, interspersed with less frequent and sometimes discontinuous elastic lamellae. The internal elastica marks the conventional boundary between intima and media. The external elastica separates the media from the adventitia. The media and adventitia of muscular arteries are approximately equal in thickness.

Arterioles contain an internal elastic lamina and one or two layers of smooth muscle cells. There is no external elastic lamina, and the adventitia consists of a thin layer of collagen and isolated elastic fibers. By the contraction of their muscle fibers, the arterioles generate the "peripheral resistance" that reduces the blood pressure at the periphery, and thereby protects the capillaries and venules.

Capillaries are vessels of small diameter (4 to 10 microns) whose wall is reduced to an attenuated endothelium surrounded by a basement membrane, a few pericytes, and connective tissue. The narrow capillary lumen allows passage of red blood cells in single file. At the level of the electron microscope, three different types of capillaries can be resolved based on the morphology of their endothelial layer:

Venules are tubes of endothelium. Small venules (up to 40-50 µm diameter) are surrounded by pericytes, contractile cells with long, branching processes that are involved in the control of blood as it flows through the microvasculature. Large venules (50-100 µm diameter) are surrounded by one or two layers of smooth muscle cells. Beyond the pericytes and the smooth muscle cells is a thin layer of connective tissue. The venular endothelium has labile junctions which "open" in inflammatory reactions under the influence of histamine, serotonin, bradykinin and other agents. The result is increased permeability and local swelling. Diapedesis, the exit of leukocytes from the vasculature, occurs also at this level of the microvasculature. You will study this process in more detail in Pathology.

Veins of small and medium size are characterized by a thin media containing only a few layers of smooth muscle cells. These vessels have a much thicker adventitia composed of collagen and occasionally some longitudinal smooth muscle fibers. In general, veins are larger in diameter and have thinner walls than arteries. The tunica adventitia makes up the greater part of the venous wall of large veins and is usually considerably thicker than the tunica media.

Other variations in the structure of blood vessels occur in certain organs in response to special functional and anatomical conditions. Some examples of special vessels include: