AJP - Gastrointestinal and Liver Physiology, Vol 242, Issue 4 364-G369, Copyright © 1982 by American Physiological Society
Unstirred water layer in canine jejunum
K. H. Ryu and E. Grim
The thickness of the unstirred water layer in in vivo-lavaged canine
jejunum has been estimated by observations on the kinetics of entrance of
[14C]inulin into the intervillus space (IVS) from the luminal fluid.
Concentrations of the inulin in the IVS at three different levels (upper,
350 micrometers; middle, 250 micrometers; and lower, 250 micrometers) were
determined as a function of duration of lavage. The concentrations rose
slowly, indicating that there was little or no convective mixing of the
fluid between the villi. After 1-2 h of lavage, mean concentrations in the
IVS were three to seven times higher than in the lavage fluid, indicating
that water absorption occurred from the IVS and that solvent drag as well
as diffusion played a role in the entrance of inulin into the IVS. Because
the concentration was always greatest at the uppermost level of the IVS,
water absorption from the IVS must have been restricted to that level.
Analysis of the data also required the inclusion of a small secretory
stream (5% of the absorptive flow) from the crypts to explain the
experimental observations. The results demonstrate that substances absorbed
into the villus tips must penetrate an unstirred layer of 500-1,000
micrometers; for those absorbed into the lateral surfaces of the villi, an
additional barrier of as much as 800 micrometers exists.
