Intracapillary events in osmotic weight transients

Abstract

In osmotic transient experiments on perfused organs the time course of the concentrations of both test and resident buffer solutes within the capillary must be known in order to give an adequate description of the underlying events leading to the observed weight changes. In accompanying papers ( G. Bloom and J. A. Johnson, 1981, Microvasc. Res. 22, 64–79, J. A. Johnson and G. Bloom, 1981, Microvasc. Res. 22, 80–92), we describe a model for osmotic transients based on the assumption that both test and buffer solute concentrations within the capillary are constant. In this paper we determine the magnitude and time course of solute concentration changes within the capillary and show the effect of inserting these values into our model in place of the previously assumed constant concentrations. In particular we show what effect these changes in solute concentration have on our estimates of capillary osmotic reflection coefficients (σ) and permeability coefficients ( P) for inulin and sodium chloride. A mathematical analysis is presented which describes the concentration as a function of time as the new solution moves through the following segments of the fluid pathway preceding the heart capillaries: (1) the tubing conduits to the heart; (2) the mixing chamber of the aortic bulb; (3) the precapillary vascular system. This analysis is combined with that of J. A. Johnson and T. A. Wilson (1966, Amer. J. Physiol. 210, 1299–1303) on the axial profile of capillary concentration caused by solute movement across the capillary wall. A mathematical analysis of the intracapillary buffering which could arise due to sieving of resident solute at the capillary wall is presented in an appendix. We find that perfusion rates similar to those that we employ in our experimental system enable us to obtain valid estimates of PA and σ. Significantly lower perfusion rates do not yield valid results.