Functional Anatomy Of The Microcirculation

Figure 1 illustrates an extensive capillary network and its associated structures (arterioles and venules) that constitute the microcirculation. Blood enters the microcirculation through arterioles, which are surrounded by a thick, continuous layer of smooth muscle. Contraction of smooth muscle reduces the internal diameter of this microvessel and consequently increases the resistance to blood flow in the entire vascular bed. This feature makes the arteriole the major resistance element in the circulation and the principal determinant of the total peripheral resistance. Arteriolar smooth muscle tone also governs the amount of pressure transmitted from arteries to veins; hence, capillary pressure falls when arterioles constrict and rises when arterioles dilate. Blood flows from arterioles into a narrower vessel, the metarteriole, which is surrounded by a discontinuous smooth muscle layer. Capillaries branch off from the metarteriole. The density of capillaries, which is an important determinant of the total area available for exchange between blood and tissue, varies significantly between organs, with the lung exhibiting the largest capillary area (3500 cm2/g) compared with muscle (100 cm2/g). The junction between the metarteriole and some capillaries is encircled by a single band of smooth muscle called the precapillary sphincter. These sphincters determine the percentage of capillaries open to blood perfusion. Although all capillaries are normally open to perfusion (and exchange) in tissues like the heart, only 20-30% of capillaries are normally open in skeletal muscle and intestine. In the latter tissues, relaxation of the precapillary sphincter allows for the recruitment of more open capillaries and hence greater transcapillary exchange. Capillaries coalesce into a venule, which

FIGURE 1 Schematic diagram of the microcirculation.

possesses a discontinuous, thin coat of smooth muscle that drains into small veins. Changes in venular smooth muscle tone can exert a significant influence on capillary exchange inasmuch as constriction of venules leads to an increased capillary pressure, whereas dilation of venules exerts the opposite effect.

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