Structure of intestinal wall
The inner lining, or mucosa, ofthe small intestine is gathered into folds and covered by a mass of fingerlike projections (villi), which increase its surface area tremendously. The epithelial cells that cover them have a so-called brushborder consisting of thousands of tiny rodlets, or microvilli, which further increase the surface area available for absorption.
A complex membrane made up of protein and lipid defines the outside edge of the microvilli. The exact molecular organization of this membrane is still not well understood, but it is believed to consist of a bimolecular layer of lipid in the center, covered or intermixed with protein. This membrane has a major role in the digestion and absorption of nutrients. The single layer of epithelial cells lining the lumen rests on a connective tissue structure (lamina propria) that contains blood and lymph vessels.
For normal absorption to occur, the substrate-for instance, glucose must enter the intestinal epithelial cell through the lipoprotein membranes and make its way across the cell, where it sometimes undergoes a chemical change. Then the substrate, glucose in this case, not only must leave on the opposite side of the cell but must pass through two additional layers of tissue before it finally enters a blood vessel. If the substrate were a fat-soluble nutrient, such as a long chain monoglyceride, the process would be similar, except that it would undergo a series of changes in the mucosal cell and enter a lacteal or lymph vessel rather than a blood capillary.