Capillary filtration and permeability coefficients calculated from measurements of interendothelial cell junctions in rabbit lung and skeletal muscle

Abstract

The depth and width of interendothelial junctions were measured in pulmonary capillaries of the adult and fetal rabbit and in capillaries of the gastrocnemius muscle of the adult rabbit. These dimensions were used to determine the resistance to filtration and diffusion of junctions in each tissue. The resistances, together with the stereologically determined values for the total length of junction per unit weight of tissue, were used to calculate maximal capillary filtration coefficients (CFC) and diffusion coefficients (CDC) for sodium and inulin under conditions in which all vessels (except those in the fetal lung) were assumed to be perfused. The calculated filtration coefficients, expressed per 100 g tissue, were 2.7, 0.8, and 0.05 ml min −1 mm Hg −1 100 g −1 for capillaries in adult lung, fetal lung, and skeletal muscle, respectively. The corresponding values for capillary surface area in each tissue were 3490, 844, and 62 cm 2 g −1, respectively. When the data were expressed per square centimeter of capillary surface area the resultant filtration and permeability coefficients were similar for all three tissues. Permeability coefficients for sodium ranged from 33 × 10 −5 to 42 × 10 −5 cm sec −1 and for inulin from 1.2 × 10 −5 to 1.9 × 10 −5 cm sec −1. The filtration and diffusion coefficients per 100 g tissue were approximately 50 times greater in adult lung than muscle and 16 to 20 times greater in fetal lung than muscle. These differences were due to the greater length of junction (open along the capillary axis) per unit depth of junction in lung compared to muscle. The effective width of the junctions in all three tissues was similar.