Active/Passive Factors in Local Blood Flow Regulation
Our recent studies have explored the possibility that blood flow may be regulated locally by mechanical factors operating at the capillary. Our evidence has indicated that flow varies with the transmural capillary pressure in heart, skeletal muscle, kidney and lung. In models, this can be shown to depend on flexible, permeable blood capillaries enclosed with extracapillary fluid and parenchymal cells in a compliant capsule. Such systems exhibit critical closing, basal tone, the hyperemias, autoregulations and other phenomena of the peripheral circulation, without need to introduce active arteriolar smooth muscle contraction. Related studies have shown that smooth muscle constriction produces remarkably large, complex effects on the lumen cross sectional area and thus even more complex effect on vascular conductance (= resistance−1This suggests that each arteriole probably can not regulate flow without the aid of a very complex local computer. It is suggested that each arteriole is either fully opened or closed. Vascular conductance may then indicate the percentage of available arterioles open at any instant. Vasoconstrictor agents would be viewed as decreasing the percentage of vessels that are open. Intermittence of closure for any single vessel would prevent prolonged ischemia of a given vascular bed. This approach could account for changes in conductance as adequately as the presently accepted belief that such vessels are capable of grading the magnitude of constriction. Related studies of such arteriolar closure on capillary conductance indicate that such an arrangement will improve the exchange of extracapillary fluids and the wash-in and wash-out of metabolic materials.