Food Absorption

After food is digested, it must be absorbed into the body through the epithelium of the digestive tract. Absorption occurs mainly in the small intestine, especially in its lower parts. Water is absorbed in the large intestine, but almost all minerals and organic substances are absorbed through the wall of the small intestine.

The intestinal mucosa forms many folds; this increases the suction surface and thereby speeds up absorption. In addition, the inner surface of these folds is covered with countless finger-like projections called intestinal villi, each of which contains a network of blood capillaries and one lymphatic capillary running in the middle. Absorption is a complex process, partly involving simple diffusion of substances from the intestinal cavity through epithelial cells into the blood or lymphatic capillaries.

Some substances are absorbed, despite the fact that their content in the blood is higher than in the intestines. The cells lining the intestine must do the work of “pumping” these substances into the blood against a concentration gradient. This process is similar to secretion, in which cells also do the work of moving substances from one area to another.

Intestinal epithelial cells form a semipermeable membrane that allows some substances, such as amino acids and glucose, to pass through, while preventing the passage of others, such as unchanged protein molecules and starch. Glucose and amino acids are absorbed into the blood capillaries and sent with the portal vein blood to the liver, from where they are subsequently distributed to the rest of the body.

Glycerol and fatty acids—products of fat digestion—enter the tissues in a different way. Bile salts play an important role in facilitating the absorption of fatty acids, monoglycerides and diglycerides (fat hydrolysis products) and other lipid-soluble substances, such as fat-soluble vitamins. As the products of lipid hydrolysis pass through the epithelial cells of the intestinal villi, fat molecules are resynthesized from them; these molecules accumulate into tiny balls that enter the lymphatic capillaries.

The contents of the intestinal lymphatic vessels eventually enter the large thoracic duct and from there into the blood, as this duct opens into the left brachial vein. In this way, the fat finally enters the bloodstream and is distributed throughout the body, but not in the same direct way as sugars and amino acids.