Permeability characteristics of colonic capillaries

Blood flow, lymph flow, lymph protein concentration (CL), lymph oncotic pressure, plasma protein concentration (CP), and plasma oncotic pressure were determined under steady-state conditions at venous pressures of 0, 10, 20, 30, and 40 mmHg in autoperfused segments of dog colon. Venous pressure elevation increased colonic vascular resistance, lymph flow, lymphatic protein flux, and the transcapillary oncotic pressure gradient, whereas the lymph-to-plasma protein concentration ratio (CL/CP) declined. The osmotic reflection coefficient (sigma d) was estimated using sigma d = 1-CL/CP when CL/CP is filtration independent (high lymph flows). For total protein sigma d = 0.85 +/- 0.02. Values of sigma d for plasma protein fractions with molecular radii ranging between 37 and 120 A increased as molecular radius increased. The results of this study suggest that 1) colonic capillaries selectively restrict macromolecules on the basis of molecular size, and 2) an increased lymph flow and transcapillary oncotic pressure gradient may play an important role in preventing interstitial edema subsequent to venous pressure elevation in the dog colon.