Interstitium-to-blood movement of macromolecules in the absorbing small intestine

The relative roles of diffusion and convection in the transport of plasma proteins across intestinal capillaries were estimated for absorbing and nonabsorbing segments of cat ileum. Diffusive, convective, and net protein fluxes were estimated with steady-state lymph and plasma protein concentrations, lymph flow, net transmucosal volume flow, and phenomenological transport equations. The results acquired indicate that convection accounts for approximately 80-90% of total net transcapillary solute movement at normal and increased capillary filtration rates. When the intestinal capillaries are converted from filtering to absorbing vessels by enhancing transmucosal water movement, the convective and diffusive protein fluxes occur in opposite directions, i.e., significant quantities of plasma proteins move from interstitium to blood by convection. These findings suggest that the magnitude of the capillary protein leakage during absorption is far greater than predicted using lymphatic protein flux changes. The massive movement of plasma proteins into and out of the mucosal interstitium during absorption may be advantageous for the removal of protein-bound nutrients (e.g., fatty acids).