Capillary and interstitial forces during fluid absorption in the cat small intestine

Abstract

A systematic steady-state analysis of the forces and membrane coefficients governing transcapillary and lymphatic fluid fluxes was performed in the cat ileum under absorptive and nonabsorptive conditions. Net transmucosal fluid flux, lymph flow, capillary pressure, blood flow, capillary filtration coefficient, and lymph and plasma oncotic pressures were measured before and during perfusion of the ileal lumen with a glucose (20 mM)-Tyrode's solution. Interstitial fluid pressure was calculated for the two conditions from measured parameters. Stimulation of mucosal fluid absorption resulted in an increase in lymph flow (which tripled), capillary filtration coefficient (which doubled), capillary pressure (which increased by 9%), interstitial fluid pressure (which doubled), and blood flow (which increased by 16%), whereas interstitial (lymph) oncotic pressure fell (by 20%). These changes converted filtering capillaries to absorbing capillaries, thereby allowing 82% of the absorbed fluid to be removed from the mucosal interstitium via the capillaries, while the lymphatics removed the remaining 18%. The results of this study indicate that hydrostatic and oncotic forces within the interstitium are primarily responsible for preventing interstitial fluid accumulation during periods of net fluid absorption in the small bowel.