Every day, about 10 liters of drink, food and GI secretions enter the alimentary canal. However, of these 10 liters, only about 1 liter or sometimes even less is able to reach the large intestine. 80% of electrolytes, majority of the water content and all foodstuffs are absorbed in the small intestines. By the time the digestive product reaches the large intestine, all that remains are traces of water, food materials that are indigestible and an abundance of bacteria.
Monosaccharides are transported by protein carriers into the epithelial cells. By way of facilitated diffusion, they then move out of the epithelial cells and enter into capillaries by way of intercellular clefts. Fructose has a special way to be absorbed, and that is by way of facilitated diffusion.
Amino acids are transported by several carries and are coupled to the active transport of sodium. Some amino acids are transported still in their short chains and what happens is they are further digested in the epithelial cells, resulting is simple amino acids before they can enter the capillary blood by way of diffusion.
During infancy, the immature intestinal mucosa absorbs whole proteins. The immune system then interprets the presence of whole proteins as antigenic and it performs its duty as the protector of the body, thereby mounting an all out attack to the protein. This then becomes evident in the child as food allergies.
Bile salts are not only involved in lipid digestion but are also active participants in the absorption of its end products. As fat digestion occurs, free fatty acids and monoglycerides are liberated. Bile salts, together with lecithin, quickly attract the liberated contents and form micelles. Found also in the core of micelles are fat-soluble vitamins and cholesterol molecules. Micelles easily diffuse into the luminal cell surface. By way of simple diffusion, lipid substances then moves out of the micelles. The lipids then float on the surface of the chyme which is beyond the reaches of the absorptive surfaces of the epithelial cells. If absorption is not completed, the lipids are lost in the feces.
Nucleic acid digestion yields a number of product including phosphate ions, nitrogenous bases and pentose sugars. These products are transported across the epithelium by way of active transport and then enter the bloodstream.
The enteric bacteria found in the large intestine are responsible making some of the vitamins K and B, which are then absorbed by the large intestine. The other dietary vitamins are absorbed in the small intestine.
Most of the water-soluble vitamins, vitamin B and C, are absorbed by way of diffusion or by way of passive transporters. The exception to this is vitamin B12. The intrinsic factor, produced by our stomach, binds to the vitamin B12. The new complex structure formed by the binding of the intrinsic factor to the vitamin B12 then binds to specific mucosal receptor in the ileum. This action then triggers active uptake by way of endocytosis.