FLOW AND ABSORPTION IN A CONTRACTING CHANNEL WITH APPLICATION TO THE HUMAN INTESTINE.

摘要

The wall movement is the sole mechanism that produces physiologically useful fluid motion inside the intestine. This movement is also believed to enhance significantly the absorption of nutrients from the chyme. Consequently, it is of major importance to find out the relationship between the wall motions and the resulting effects on the uptake of nutrients. This study is a first attempt to investigate both experimentally and analytically the influence of boundary movements upon the transport of substances through the walls of the intestine. Two types of boundary movements, which are considered to be important in intestinal mechanics, were analyzed in this study: the standing transverse contractions and the combination of transverse and longitudinal contractions.;In the theoretical analysis, a Fourier series was used to describe the wall motions in the physical model. An approximate solution of the flow field was derived for the low Reynolds number condition which is of primary concern for the small intestine. The flow field thus obtained was then used in the governing equation for the unsteady diffusion-convection mass transfer. An alternate-direction-method was used to solve the governing equation which is of variable coefficients. The results show very good agreement with those experimentally obtained.;Results of this study indicate that the inertia-free approximation is valid up to a Reynolds number equal to five. The standing transverse contraction, which does not induce permanent convective mixing, only moderately enhances the absorption function. With the addition of a longitudinal propagative contraction, the resultant progressive transverse contraction significantly improved the rate of absorption. This is due to permanent convective mixing effects.;These results demonstrate the relative importance of the boundary movements for the human intestine. To further investigate more complex wall motions, the method adopted in this study will be advantageous since many other motions can be represented easily by the same procedure.;In the experimental part of the study, a concentration probe which measures the conductivity of the fluid was constructed and used. A simplified model involving a system of cams was designed to generate the desired wall movements. The experiments were conducted in such a way that the results could be obtained for processes with and without the wall motions. These results were then used to compare with those obtained from the analysis.