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STRUCTURE OF THE BLOOD VESSELS
Arteries. The arteries are a series of vessels carrying blood from the heart to the capillaries. The caliber of the arteries decreases gradually when followed toward the capillaries. The largest arteries, the aorta and the pulmonary artery, have a lumen about 25 mm. (1 inch) in diameter and the smallest arteries about 0.3 mm. in diameter, or smaller. Three layers can be distinguished in the walls of every artery: (1) the inner coat, tunica intima, (2) the intermediate coat or tunica media and (3) the external coat or tunica adventitia. The tunica intima is composed of a layer of endothelial cells and a conspicuous band of elastic tissue. In the larger vessels there is a fine network of connective tissue between the endothelial and elastic layers. The intermediate coat or tunica media consists mainly of smooth muscle and elastic tissue. Smooth muscle tissue predominates in the small arteries and elastic tissue in the large arteries, such as the aorta. The tunica adventitia is made up of collagenous and elastic fibers in loose arrangement. In small and medium-sized arteries the tunica adventitia is as thick as, and sometimes thicker than, the media but in arteries of large size the external coat is very thin. Veins. The veins comprise a system of vessels which collect the blood from the capillaries and return it to the heart. The structure of veins resembles that of the arteries and usually the three layers, intima, media and adventitia can be distinguished. The veins, however, have thinner walls than the arteries and they are less elastic. This is due to the poorly developed middle layer which is very thin and contains very little smooth muscle and elastic tissue. Some veins are equipped with valves. They appear as semilunar pockets on the inner surface of the vein and are placed with their free edges in the direction of the blood flow, so that flow toward the heart is not impeded but flow away is prevented. Valves are more numerous in the veins of the lower extremities. Capillaries. The capillaries lie between the terminal branches of the arteries and the beginning of the veins. There the blood bed is tremendously expanded, each small artery branching into a number of capillaries which interlace to form an intricate network. The capillaries are extremely fine thin tubes, the walls consisting of a single layer of flat endothelial cells. The diameter of the capillaries in various parts of the body varies slightly but is closely related to the size of the red blood corpuscle, averaging about 8 microns. When viewed through the microscope, the red blood cells shine through the transparent walls of the capillary and usually they are seen passing in single file. The capillaries are the functional units of the circulatory system. It is through their walls that exchanges take place between the blood and tissues. The blood flows along rapidly in the arteries and veins but when it reaches the capillary areas, where it flows into many tubes of small caliber, it is greatly slowed down and the stream passes leisurely through the network of vessels. This gives time for the exchange of materials to occur.
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Text: Heart Grammar: Infinitive.
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