Blood Vessels Health Article

Definition

Blood vessels compose a continuous system of channels through which blood transports oxygen and nutrients to and waste materials from all body tissues.

Structure

All blood vessels (except capillaries) share a similar three-layered structure. The innermost layer, called the tunica intima, is composed of a monolayer of endothelial cells called the endothelium. The tunica intima helps to restrict the entry of substances into the vascular wall, control blood vessel diameter, and regulate coagulation. The hollow center of a blood vessel is called the lumen and is the space through which blood flows.

The middle layer is called the tunica media and is separated from the tunica intima by a sheath of high-flexible material called the internal elastic lamina. The tunica media is composed of a circular arrangement of smooth muscle cells, collagen, and elastic fibers; it composes the bulk of the wall of most arteries but in veins is thinner and contains fewer smooth muscle cells. Smooth muscle contains contractile elements that are responsible for contraction (vasoconstriction) and relaxation (vasodilation). The tunica media, therefore, imparts strength, elasticity, and contractile abilities to the vessel wall.

Surrounding the tunica media is the tunica adventitia (the two layers are separated by the external elastic lamina). This outermost layer contains a matrix of collagen and elastic fibers that support fibroblasts (cells that secrete the fibrous proteins collagen and elastin), nerves, and vasa vasorum (small blood vessels that supply the walls of large arteries and veins with oxygen and nutrients).

Arteries and arterioles

Arteries are blood vessels that carry blood away from the heart. Arterial blood is oxygen-rich, with the exception of blood carried by the pulmonary artery from the heart to the lungs to be oxygenated. The aorta is the largest artery in the human body and originates at the left ventricle of the heart. This vessel and its major branches (the common carotid, common iliac, subclavian, and brachiocephalic arteries) are called elastic arteries because they expand and recoil in response to the pulsing flow of blood and to changing blood volume.

The elastic arteries branch to become muscular arteries, vessels with thick walls that transport blood to specific organs. Muscular arteries give rise to resistance vessels; these include small arteries and arterioles. As arteries become smaller, their walls become thinner and are composed of less collagen and elastin. The walls of small arteries have multiple layers of smooth muscle cells, while arterioles have only one or two. Resistance vessels are thus less stretchy but more active in regulating the flow of blood into capillary beds.

Anastomoses are formed where arteries and arterioles merge to provide alternative channels for blood delivery. They provide collateral circulation in the event that an artery becomes occluded (blocked).

Exchange vessels

Exchange vessels include capillaries and postcapillary venules. The walls of capillaries are composed of only a tunica intima (a thin layer of endothelial cells). The average diameter of the lumen is just large enough to allow erythrocytes (red blood cells) to pass through in single file. Exchange vessels are the site where gases, nutrients, and wastes are exchanged between blood and surrounding tissues.

There are three major type of capillaries: continuous, fenestrated, and discontinuous. Continuous capillaries are the most abundant type in the human body and are found in skin, muscle, lungs, and the central nervous system. They have low permeability and therefore allow only limited passage of substances across the capillary wall. Fenestrated capillaries are much more permeable than continuous capillaries; their walls contain circular pores or fenestrae closed by a thin diaphragm. Discontinuous capillaries, also called sinusoids, have gaps between endothelial cells that are large enough to allow even erythrocytes to pass through the capillary wall. They are found in the liver, spleen, and bone marrow, as well as some endocrine glands.

The capillary bed is a network of capillaries that connect arterioles with venules; there are typically 10 to 100 capillaries per bed. Arterioles give rise to either capillaries or metarterioles, vessels that are wider than true capillaries and directly connect arterioles to venules. True capillaries branch off arterioles or metarterioles and are encircled at their origin by the precapillary sphincter, permitting the regulation of blood flow into the capillary. Arteriovenous (A-V) shunts are anastomoses that bypass the capillary bed completely; they are frequently seen in tissues that require increased blood flow.