Background/aims: The flow rate of fluid through the proximal small intestine varies widely under normal physiological conditions. The aim of this study was to assess the effect of changes in flow rate on the passive permeability of the aqueous paracellular pathway of the human jejunum.
Methods: Normal subjects were studied in vivo during constant perfusion of 30-cm loops of jejunum at flow rates of 5, 10, or 20 mL/min. The permeability ratio of L-xylose/urea was used to assess apparent permeability of the mucosa and to calculate the average pore radius of the aqueous pathway for passive diffusion.
Results: Increasing jejunal flow rate from 5 to 20 mL/min significantly decreased the L-xylose/urea permeability ratio from 0.35 to 0.23 and decreased average calculated pore radius of the diffusion pathway from 13 A to 8 A.
Conclusions: Increases in flow rate in the normal physiological range decrease the estimated pore size of normal healthy jejunal mucosa. Because increasing flow rate is known to increase exposure of luminal fluid to the intervillus space, the results of this study are best explained by postulating that cells lining the sides of villi are less permeable than cells lining the villus tips.