Correlation of fluid exchange in the microcirculation and macrocirculation.

Accurate, quantitative information relating to hydraulic conductivity (Lp) of the capillary wall in any organ of tissue is essential as the first step toward understanding fluid homeostasis. Any perturbation which permanently alters fluid balance will ultimately lead to death and destruction of tissues in the living organism. This report summarizes some recent information relating to Lp measurements in single capillary and whole organ experiments. It is shown that the single capillary and whole organ methods do not yield similar quantitative information and some possible reasons for this are discussed. It is hypothesized that capillary membrane permeability is governed by dynamic processes which may change in response to both the metabolic requirements of the organ tissue cells and overall metabolic homeostasis of the organism. It follows from this hypothesis that fluid balance in tissues is a passive process determined solely by the Starling forces, the functional significance of which is to provide an aqueous medium to aid the diffusion of nutrient substrate from blood to tissue cells.