ABSTRACT
Conversely, glucose can reach the epithelium via di¬usion in laminar ow (Figure 14.1) [3]. If the di¬usion rate of glucose in the lumen is lower than its transepithelial absorption rate, the overall absorption rate of the glucose should be proportional to either its di¬usion rate in the lumen or its membrane transport rate. ´e slower of these two factors is the rate-limiting factor for the overall absorption process. Considering the mentioned short-chain fatty acid gradient and heterogeneous distribution of microbiota across the intestinal lumen [6,7], the di¬usion rate in the lumen should be slower than the membrane transport rate. ´erefore, the di¬usion rate of glucose in the lumen should correlate with the overall absorption rate in laminar ow (Equation 14.2):
( ) ( ),Overall absorption rate Diffusion rate in lumen= b (14.2)
where b is a constant. ´e ow behavior of digesta in the lumen can be estimated using the Reynolds
number, which expresses the ratio of inertial forces to viscous forces in a uid [8]. ´e inertial force is the tendency of the uid to stay in motion or at rest unless acted upon by an outside force [9]. ´e viscous force is an internal property of a uid that o¬ers resistance to ow [9]. ´e Reynolds number is used to determine whether a ow will be dominated by inertial or viscous forces, i.e., whether the ow is turbulent or laminar. A Reynolds number below 2300 indicates that viscous force predominates over inertial force to keep the ow laminar (Figure 14.2a), which results in poor micromixing along the transverse axis. Conversely, a Reynolds number exceeding 2300 indicates that inertial force dominates and that the ow has become turbulent [8] (Figure 14.2b), which completely mixes digesta at the molecular level [4].
