Analysis of lymphatic protein flux data. V. Unique PS products and sigma dS at low lymph flows.

The selectivity of the capillary membrane to protein (osmotic reflection coefficient, sigma d) can be measured at high transcapillary volume flow when the capillary membrane can be considered as a true sieve. However, the diffusive capacity of the membrane (permeability-surface area product, PS) for macromolecules has not been directly measured, only estimated by assuming that transcapillary volume flow was zero. Based on unique properties of the Peclet number, a parameter that describes the ratio of solute convective flux relative to diffusive capacity, we have developed three new techniques using lymph protein fluxes to estimate a unique PS product that is independent of transcapillary fluid flux. Two of these techniques require a measure of sigma d when the ratio of protein concentration in lymph relative to plasma is equal to (1- sigma d), which occurs at high capillary filtration rates. However, the third method allows both sigma d and the PS product to be determined at relatively low lymph flow rates, eliminating the need for high capillary pressures to determine sigma d. For each protein, these techniques yield an estimate of PS and sigma d for the total membrane. However, by analysis of several different sized proteins and estimation of small- and large-pore volume flows, sigma d and PS can be determined separately for the small- and large-pore pathways. These techniques for estimating sigma d and PS were evaluated by modeling the total solute flux of albumin and immunoglobulins G and M in a heteroporous membrane.