Transcapillary exchange in relation to capillary circulation

Transcapillary exchange of diffusible solutes depends on capillary blood flow, Q; capillary permeability, P; and capillary surface area, S. In a single capillary, the extent of equilibration of a given solute depends on the ratio of Q, to the product of P and S. In a microvascular bed consisting of many capillaries, equilibration depends on the fraction of them which are open to blood flow at any time and on the distribution of Q/PS ratios in the open capillaries. Both these characteristics are subject to control by vascular smooth muscle, particularly by the precapillary sphincters. Vasomotor mechanisms have been shown experimentally to exert a wide range of effective control over blood-tissue transport. In skeletal muscle, effective PS measured with ⁴²K or ⁸⁶Rb may be increased 8-fold from maximum nervous vasoconstriction to optimum metabolic vasodilatation. Most probably, these changes are due to differences in functional capillary surface area and of blood flow distribution relative to permeability and surface area. The extent to which variations in permeability itself can contribute to control of transcapillary exchange is not known.