Absorption

     The process by which nutrients are transported from the lumen of the gastrointestinal tract to the blood or lymphatic system. Absorption of most nutrients occurs predominantly in the jejunum. Absorption of intact macromolecules is very limited. Most are degraded into their constituents by digestive enzymes in the intestinal lumen: proteins to amino acids and small oligopeptides; glycogen to maltose, isomaltose and small oligosaccharides; triglycerides to fatty acids, 2-monoglycerides and glycerol. Further degradation of proteins and carbohydrates occurs at the brush border surface under the influence of a large number of specific enzymes for degradation to their monoconstituents, amino acids (small amounts of peptides may pass to the blood) and the hexoses glucose, fructose and galactose (fructose is converted to glucose in the intestinal cells before being transferred to the blood). Degradation products from lipids are emulsified by bile salts and lecithin and organized in micelles which diffuse through the unstirred water layer to the membrane of the brush border, where the components are absorbed except the bile salts.

     Absorption of macromolecules can occur in specific instances; for example, absorption of immunoglobulins from colostrum in newborn mammals is performed by pinocytosis, mainly in the ileum.

     Absorption of some minerals and of degradation products from microbial fermentation, such as short-chain fatty acids (SCFA), also takes place in the large intestine. In the horse, up to 70% of the absorbed energy is absorbed as SCFA in the colon. In ruminants, absorption of these products mainly takes place in the fore-stomach.

     Little water is absorbed from the stomach, but it moves freely across the mucosa in both directions in the small intestine and large intestine and generally the osmolality in the intestinal lumen is close to that of plasma. In the colon, sodium is pumped out and water moves passively with it.