Blood Vessels

The central opening of a blood vessel, the lumen, is surrounded by a wall consisting of three layers:
  • The tunica intima is the inner layer facing the blood. It is composed of an innermost layer of endothelium (simple squamous epithelium) surrounded by variable amounts of connective tissues.

  • The tunica media, the middle layer, is composed of smooth muscle with variable amounts of elastic fibers.

  • The tunica adventitia, the outer layer, is composed of connective tissue.

The cardiovascular system consists of three kinds of blood vessels that form a closed system of passageways:

  • Arteries carry blood away from the heart. The three kinds of arteries are categorized by size and function:

    • Elastic arteries (conducting arteries) are the largest arteries and include the aorta and other nearby branches. The tunica media of elastic arteries contains a large amount of elastic connective tissue, which enables the artery to expand as blood enters the lumen from the contracting heart. During relaxation of the heart, the elastic wall of the artery recoils to its original position, forcing blood forward and smoothing the jerky discharge of blood from the heart.

    • Muscular arteries (conducting arteries) branch from elastic arteries and distribute blood to the various body regions. Abundant smooth muscle in the thick tunica media allows these arteries to regulate blood flow byvasoconstriction (narrowing of the lumen) or vasodilation (widening of the lumen). Most named arteries of the body are muscular arteries.

    • Arterioles are small, nearly microscopic blood vessels that branch from muscular arteries. Most arterioles have all three tunics present in their walls, with considerable smooth muscle in the tunica media. The smallest arterioles consist of endothelium surrounded by a single layer of smooth muscle. Arterioles regulate the flow of blood into capillaries by vasoconstriction and vasodilation.

  • Capillaries are microscopic blood vessels with extremely thin walls. Only the tunica intima is present in these walls, and some walls consist exclusively of a single layer of endothelium. Capillaries penetrate most body tissues with dense interweaving networks called capillary beds. The thin walls of capillaries allow the diffusion of oxygen and nutrients out of the capillaries, while allowing carbon dioxide and wastes into the capillaries.

Below is a list of the different types of capillaries:

  • Metarterioles (precapillaries) are the blood vessels between arterioles and venules. Although metarterioles pass through capillary beds with capillaries, they are not true capillaries because metarterioles, like arterioles, have smooth muscle present in the tunica media. The smooth muscle of a metarteriole allows it to act as a shunt to regulate blood flow into the true capillaries that branch from it. The thoroughfare channel, the tail end of the metarteriole that connects to the venule, lacks smooth muscle.

  • True capillaries form the bulk of the capillary bed. They branch away from a metarteriole at its arteriole end and return to merge with the metarteriole at its venule end (thoroughfare channel). Some true capillaries connect directly from an arteriole to a metarteriole or venule. Although the walls of true capillaries lack muscle fibers, they possess a ring of smooth muscle called a precapillary sphincter where they emerge from the metarteriole. The precapillary sphincter regulates blood flow through the capillary. There are three types of true capillaries:

  • Continuous capillaries have continuous, unbroken walls consisting of cells that are connected by tight junctions. Most capillaries are of this type.

  • Fenestrated capillaries have continuous walls between endothelial cells, but the cells have numerous pores (fenestrations) that increase their permeability. These capillaries are found in the kidneys, lining the small intestine, and in other areas where a high transfer rate of substances into or out of the capillary is required.

  • Sinusoidal capillaries (sinusoids) have large gaps between endothelial cells that permit the passage of blood cells. These capillaries are found in the bone marrow, spleen, and liver.
  • Veins carry blood toward the heart. The three kinds of veins are listed here in the sequence they occur regarding the flow of blood back to the heart:
    • Postcapillary venules, the smallest veins, form when capillaries merge as they exit a capillary bed.     Much like capillaries, they are very porous, but with scattered smooth muscle fibers in the tunica       media.

    • Venules form when postcapillary venules join. Although the walls of larger venules contain all three layers, they are still porous enough to allow white blood cells to pass.

    • Veins have walls with all three layers, but the tunica intima and tunica media are much thinner than in similarly sized arteries. Few elastic or muscle fibers are present. The wall consists primarily of a well‐developed tunica adventitia. Many veins, especially those in the limbs, have valves, formed from folds of the tunica intimathat prevent the backflow of blood. If these valves fail to close properly, varicose veins may occur.

Many regions of the body receive blood supplies from two or more arteries. The points where these arteries merge are called arterial anastomoses. Arterial anastomoses allow tissues to receive blood even after one of the arteries supplying blood has been blocked.

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