Symposium Proceedings

Abstract

Starting with the earliest in vivo studies of the microcirculation, observers have been impressed with the variability and heterogeneity of microcirculation blood flow. The question I should like to address is how heterogeneity of microvascular perfusion influences transport of diffusible solutes, and how control of heterogeneity may contribute to regulation of blood-tissue exchange. Available exchange vessel surface area and inter-exchange vessel distances are controlled at the arteriolar level by the number of vessels open to the supply of arterial blood. When metabolic requirements are low, only a fraction of the exchange vessels in a vascular network are perfused; as metabolic activity increases, more exchange vessels are recruited. However, the efficiency with which available exchange vessel surface is utilized for diffusion of a given solute depends on the distribution of individual blood flow/permeability surface area ratios (Q/PS) in the network. Maximum efficiency (full utilization of available PS) is achieved when Q/PS is uniform.In vivo observations of red blood cell velocities show wide variation among individual exchange vessels of the same class (capillaries, postcapillary venules); presumably the same is true of plasma velocities. Flow velocity is only one of the factors that determines local transport; the critical parameter is the product of velocity and vessel radius divided by vessel length times solute permeability (v. r/ 1 P). The impression I get from published intra-vital microscope data is that this ratio may vary even more widely than velocity alone. Physiological measurements of solute transport in whole organs provide indirect support for broad heterogeneity of perfusion, at least in low metabolic states. In resting skeletal muscles the apparent (measured) PS products for 86Rb and 51Cr EDTA increase with increasing perfusion rate, approaching limiting values at high flow rates, as predicted for a heterogeneity perfused assemblage of exchange vessels. Estimated coefficients of variation (cv = S.D./mean) of Q/PS or v. r/ 1 P lie in the range 07-09.In heterogeneously perfused organs and tissues, the degree of heterogeneity (cv of Q/PS) and the total blood flow are potentially important secondary regulators of capillary transport, because they determine the efficiency with which the primary controlling factors, total blood flow and available exchange vessel surface area, are utilized (note that blood flow acts both directly and indirectly). Efficiency, defined as the ratio of apparent PS to the maximum PS at uniform perfusion, is inversely related to the degree of heterogeneity and directly related to the mean flow velocity. There is conflicting evidence in the literature concerning the degree to which alteration of Q/PS heterogeneity is involved in adaptive control of diffusion exchange. However, even with constant total PS and fixed heterogeneity, an increase in total blood flow can decrease the influence of heterogeneity on transport and thus contribute substantially to Berlin 33, Ger...