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Metabolic models of microcirculatory regulation.

The functions and integrity of body tissues are critically dependent on an adequate oxygen supply. Because the transport of oxygen to the cells is intimately linked to the microcirculation, the concept of microcirculation-metabolism coupling has received much attention. In essence, the metabolic theory of intrinsic control of the microcirculation states that microvascular tone is locally modulated to maintain adequate oxygen levels in the parenchymal cells. We propose a two-component control system for the regulation of tissue O2 delivery in accordance with metabolic needs. A precapillary sphincter control mechanism maintains tissue PO2 by governing the number of perfused capillaries. Functional capillary density in turn determines surface area available for diffusion and capillary-to-cell diffusion distance. On the other hand, the arteriolar control system modulates local blood flow in accordance with parenchymal O2 utilization and thereby minimizes changes in capillary PO2 when the O2 availability/demand ratio is decreased. We propose that the precapillary sphincters are more sensitive to changes in tissue PO2 than are the flow-regulating arterioles. Consequently, for mild stresses, adequate tissue oxygenation is maintained mainly by precapillary sphincter control of diffusion parameters without the need for changes in blood flow. However, as metabolic stresses become greater, blood flow regulation becomes the dominant factor in the control of tissue O2 delivery. Thus, by working in concert, the local mechanisms regulating microvascular resistance and effective capillary density provide a wide margin of safety against the development of cellular hypoxia.