Modeling the effect of wall movement on absorption in the intestine

Intestinal wall movements may influence intestinal absorption by inducing flow in the luminal volume. Direct evidence for this is scant and weak. Such evidence was sought by mechanical and analytical modeling. Wall motions resembling peristaltic ring contractions were induced in dialysis tubing filled with a concentrated salt solution and immersed in tap water. The disappearance of solute from the inside of the tube was monitored with conductivity probes and correlated with various kinds of wall motions. In analytical modeling, equations representing wall motions, flow, and mass transfer were constructed, with the assumptions that ring contractions are axisymmetric, the wall is totally compliant, the Reynolds' number is low, the fluid volume is conserved, the no-slip condition exists at the luminal surface, and inertial flow is negligible. In the mechanical model, wall motions caused a 30-35% increase in absorption that was matched in the corresponding analytical model. The analytical model demonstrated that the effect is progressively greater for nonpropagative stationary contractions, asymmetric progressive contractions, and symmetric progressive contractions, the increase being 33, 70, and 100%, respectively.