27.33 : Mechanical and Chemical Digestion in the Small Intestine

The small intestine plays a crucial role in our digestive system, performing both mechanical and chemical digestion.

Mechanical digestion in the small intestine involves movements such as segmentations and migrating motility complexes (MMCs), primarily controlled by the myenteric plexus. Segmentations are localized contractions occurring in areas of the intestine distended by chyme—a mixture of partially digested food. These contractions mix chyme with digestive juices, facilitating absorption by bringing food particles into contact with the intestinal mucosa. After most of a meal has been absorbed, intestinal wall distension decreases, segmentation ceases, and peristalsis begins. The type of peristalsis in the small intestine, known as a migrating motility complex, begins in the distal portion of the stomach (pylorus) and pushes chyme forward along a short intestinal stretch.

Chemical digestion in the small intestine involves the collective effort of pancreatic juice, bile, and succus entericus (intestinal juice). It completes the digestion of carbohydrates, proteins, and lipids that begin in the stomach.

Carbohydrate digestion involves enzymes like salivary amylase and pancreatic amylase breaking starches into smaller fragments. The brush-border enzyme α-dextrinase further breaks down dextrins by releasing one glucose unit at a time. Meanwhile, disaccharides—sucrose, lactose, and maltose—are broken down into monosaccharides (glucose, fructose, and galactose) by sucrase, lactase, and maltase, respectively.

Protein digestion starts in the stomach with pepsin fragmenting proteins into peptides. This process continues in the small intestine with enzymes in pancreatic juice—trypsin, chymotrypsin, carboxypeptidase, and elastase—further breaking proteins into peptides. Protein digestion concludes in the brush border, where aminopeptidase and dipeptidase convert peptides into individual amino acids, which are then absorbed by enterocytes..

Lipid digestion predominantly occurs in the small intestine, with pancreatic lipase breaking down triglycerides into fatty acids and monoglycerides. A minor amount of lipid digestion happens in the stomach via lingual and gastric lipases.

Lastly, nucleic acids are digested by ribonuclease and deoxyribonuclease present in pancreatic juice. These enzymes break down RNA and DNA, respectively, into nucleotides. These nucleotides are further degraded by brush-border enzymes called nucleosidases and phosphatases into pentoses, phosphates, and nitrogenous bases.