Histology of blood vessels
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Blood vessels, namely arteries and veins, are composed of endothelial cells, smooth muscle cells and extracellular matrix (including collagen and elastin). These are arranged into three concentric layers (or tunicae): intima, media and adventitia.
- the intima (or tunica intima)
- inner layer abutting the vessel lumen, and the thinnest layer
- composed of a single layer of endothelial cells and a small amount of subendothelial connective tissue
- is separated from the media by a dense elastic membrane called the internal elastic lamina
- the media (or tunica media)
- sandwiched between the intima and adventitia, and the thickest layer
- provides structural support, vasoreactivity and elasticity
- composed of smooth muscle cells, elastic fibers and connective tissue, which vary in amount depending on the type of vessel
- smooth muscle cells contract (vasoconstriction) or relax (vasodilatation), which is controlled by autonomic nerves (nervi vasorum) and local metabolic factors
- elastic fibers allow the vessel to expand with systole and contract with diastole, thereby propelling blood forward
- the media is separated from the adventitia by a dense elastic membrane called the external elastic lamina
- the adventitia (or tunica adventitia)
The intima and inner part of the media are nourished by diffusion of oxygen and nutrients from blood in the lumen, and the adventitia and outer part of the media are nourished by vasa vasorum.
The walls of arteries are thicker than that of veins to withstand pulsatile flow and higher blood pressures. As arteries become smaller, wall thickness gradually decreases but the ratio of wall thickness to lumen diameter increases (i.e. relative lumen size decreases).
Arteries are divided into three types according to size and function. The constituents of the media of these vessels differ in their relative amounts accordingly.
- large elastic arteries (aorta, large aortic branches [e.g. innominate, subclavian, common carotids, iliacs] and pulmonary arteries): the media is abundant in elastic fibers that allow it to expand with systole and recoil during diastole, thereby propelling blood forward
- medium-sized muscular arteries (other aortic branches, e.g. coronary and renal arteries): the media is abundant in smooth muscle cells that vasoconstrict or vasodilate, thereby controlling lumen diameter and regional blood flow
- small arteries and arterioles (in the substance of organs and tissues): the media is abundant in smooth muscle cells that vasoconstrict or vasodilate; in vessels of this size, smooth muscle contraction causes dramatic changes in lumen diameter, thereby controlling systemic blood pressure as well as regional blood flow
Capillaries connect arterioles with venules. They consist only of a single layer of endothelial cells on a basement membrane. There is no media or adventitia. The diameter is just wide enough for passage of a red blood cell, therefore flow is very slow. These features facilitate exchange of oxygen, nutrients and other substances between blood and tissues.
Post capillary blood flows into venules and then into progressively larger veins.
Compared to arteries, veins have larger diameters and thinner walls. They therefore have larger lumens and contribute capacitance to the circulation, holding approximately two thirds of all circulating blood.
The intima and adventitia are similar in structure and function to arteries but the media is much thinner due to significantly less smooth muscle and elastic tissue. Veins therefore do not have the same capacity for elastic recoil and vasoconstriction as arteries. Blood is propelled forward by contraction of surrounding muscles and pressure gradients created during inspiration and expiration. Reverse flow is prevented by the presence of venous valves.
The flaccid walls of veins predispose them to compression and penetration by tumor and inflammatory processes.