Physiological measurement of luminal stirring in the dog and human small bowel

The resistance to absorption resulting from poor stirring of luminal contents (R(Lum)) is considered to be equivalent to an unstirred layer of greater than 600 μm in the human small intestine. We measured R(Lum) in the jejunum of conscious dogs by assessing the absorption rate of two rapidly adsorbed probes, glucose, and [¹⁴C]warfarin. When R(Lum) was expressed as an unstirred layer, the maximal thickness of the unstirred layer (assuming negligible epithelial cell resistance) was only ~35 and 50 μm for perfusion rates of 26 and 5 ml/min, respectively. Maximal unstirred layer thickness for the human jejunum, calculated from previous studies of glucose absorption, yielded a mean value of only 40 μm (range: 23 to 65 μm). Since epithelial resistance appears to be negligible during absorption of low concentrations of glucose, the maximal unstirred layer of 40 μm should be close to the true value for glucose in the human small intestine. We conclude that the unstirred layer for rapidly absorbed compounds in dogs and man are less than one-tenth of previously reported values, but this layer still may remain the rate limiting step in absorption of rapidly transported compounds.