Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

The Science of Viscera (Splanchnology)
Digestive System
Food Products and Nutrients — Absorption

Absorption takes place predominantly in the Small Intestine. Digestion products—Amino Acids and their components, Monosaccharides, emulsified fats, as well as Water-soluble salts and Vitamins—are absorbed into the Blood and Lymphatic capillaries of the small intestinal mucosa. Water enters the body with food and through secretions produced by the digestive glands. Numerous mucosal villi and epithelial microvilli of the small intestine create an enormous absorptive surface area (approximately 200 m²). Thanks to their contractile and relaxing smooth Muscle Cells, these villi function like miniature suction pumps. Consequently, absorption is an active process that requires Energy Expenditure by the epithelial cells. The enterocytes allow digested Proteins (amino acids) and simple sugars (glucose) to pass from the intestinal lumen into the bloodstream while barring large undigested proteins and complex sugars (Polysaccharides). Fats, having been digested (broken down) in the intestine into glycerol and Fatty acids, are absorbed into the lymphatic capillaries. On their way into the lymphatic capillaries, glycerol and fatty acids recombine within the epithelial cells to form microscopic, water-soluble fat droplets. Fatty acids are rendered soluble with the help of Bile acids, whereas glycerol is readily soluble in water. In the absence of bile acids in the intestine—such as in Liver or Gallbladder diseases—fat absorption fails to occur, and fats are excreted in the feces.

Water and dissolved salts are absorbed in the Large Intestine. Certain medications, such as Glauber's salt (sodium sulfate and other sulfonamide drugs), are poorly absorbed through the epithelial lining of the intestinal mucosa. When such drugs are administered, the osmotic pressure in the gut rises sharply; water is retained in the intestine, stretching its walls, enhancing peristalsis, and producing a laxative effect. Alcohol and certain medicinal substances—such as hypnotics (barbiturates) and aspirin (acetylsalicylic acid)—are absorbed in The Stomach. Nutrients are not absorbed in the stomach because they have not yet been sufficiently digested. In the Oral Cavity, the medication nitroglycerin is readily absorbed through the mucous membrane.