Mechanical and Chemical Digestion in the Small Intestine

Segmentations are localized contractions of the small intestine triggered by distention of the intestinal wall from chyme, a mixture of partially digested food. These contractions mix chyme with digestive juices and bring food particles into contact with the intestinal mucosa, facilitating nutrient absorption. Segmentations occur throughout the small intestine while food is being processed.

The migrating motility complex (MMC) is a type of peristalsis that begins in the distal portion of the stomach and pushes chyme forward along the small intestine. MMC starts once absorption is mostly complete and intestinal wall distension decreases. This coordinated wave of contractions propels remaining chyme toward the large intestine.

Pancreatic juice contains proteases including trypsin, chymotrypsin, carboxypeptidase, and elastase that break proteins into smaller peptides. These peptides are then digested into individual amino acids by brush-border peptidases located on the intestinal mucosa. This two-stage process completes protein digestion begun in the stomach.

Bile emulsifies large lipid droplets into smaller micelles, increasing their surface area for enzyme action. Pancreatic lipase then breaks down these micelles into monoglycerides and fatty acids, which can be absorbed by enterocytes. This emulsification step is essential because it enables efficient lipid digestion and absorption.

Pancreatic amylase breaks down starch into smaller fragments, while brush-border enzymes including α-dextrinase and disaccharidases complete carbohydrate digestion. Sucrase, lactase, and maltase convert disaccharides into absorbable monosaccharides like glucose, fructose, and galactose. These monosaccharides are then absorbed by enterocytes lining the intestinal wall.

Ribonuclease and deoxyribonuclease in pancreatic juice digest RNA and DNA into nucleotides. Brush-border enzymes called nucleosidases and phosphatases further break down nucleotides into pentoses, phosphates, and nitrogenous bases. These smaller components are then absorbed by the intestinal epithelium for cellular use.

The small intestine uses intestinal juice, pancreatic juice, and bile to complete chemical digestion of macromolecules. These secretions work collectively to digest proteins, lipids, carbohydrates, and nucleic acids that began breaking down in the stomach. The absorption of nutrients occurs as these substances are digested into their smallest components.