Permeability and reflection coefficients from osmotic transients—Extravascular factors

Abstract

F. F. Vargas and J. A. Johnson in 1964 and 1967 ( J. Gen Physiol. 47, 667–677; Amer. J. Physiol. 213, 87–93) published a technique for obtaining capillary permeability and reflection coefficients from the rate of weight loss of an organ undergoing an osmotic transient. The rate of weight loss was plotted versus time on semilogarithmic paper. From the intercept an estimate of the capillary reflection coefficient could be made. From the slope an estimate of the permeability coefficient could be made. We have examined this technique using a model developed by G. Bloom and J. A. Johnson (1981, Microvasc. Res. 22, 64–79). In this paper we examine the effects of factors such as interstitial and cell buffering, solvent drag, interstitial elasticity, and cell water permeability while we maintain the intracapillary concentrations constant. In a subsequent paper we relax this restraint and allow inlet and intracapillary conditions to influence the transient and thus evaluate the role they play in an osmotic weight transient. In the present paper we have found the intercept technique for obtaining the reflection coefficient to be valid. However, the slope-permeability technique has to be modified in some cases. For highly water-soluble molecules the size of inulin, or larger, an elastic factor correction must be made. This occurs because the tendency for a large transfer of fluid from the interstitium to the capillaries is opposed by an interstitial pressure drop which influences the time course of the entire process and also gives rise to a minimum in the weight vs time curve. A simple analytical model is given which agrees very well with the full computer model for these larger compounds. In the case where the test solute is identical in properties to the extracellular buffer (e.g., NaCl), the slope (rate constant) must be modified by using the total water (cellular plus extracellular) in the denominator instead of the interstitial volume. An analytical model for this type of transient is given which also agrees very well with the computer model for NaCl.