Dynamics of transcapillary fluid exchange

ABSTRACT Fluid balance at the capillary level has been simulated with an analogue computer program, based on experimental data on regional differ-ences in capillary permeability, surface areas, and hydrostatic pressures. The program takes into account fluid and protein fluxes into and out of the inter-stitial space. Solutions are obtained for tissue hydrostatic pressure, tissue fluid osmotic pressure, interstitial space volume, and lymph flow. Simulation of a variety of physiological experiments and clinical disease states has yielded rea-sonable agreement between experimental data and data obtained by computer analysis. Dilution of the interstitial plasma protein pool with a consequent reduc-tion of its oncotic pressure appears to be a major factor, which prevents edema unless plasma oncotic pressures are reduced by 10-15 mm Hg or, alternatively, venous pressures are elevated by a similar amount. The computer analysis in all instances yields positive values for tissue pressure, in agreement with experi-mental data obtained by needle puncture. The negative tissue pressures ob-served in subcutaneous capsules can be reproduced in the computer program