Physiology of the Gastrointestinal System II: Digestion and Absorption

In the stomach, food is transformed into a semi-fluid mass called chyme through mechanical churning and chemical digestion. The stomach secretes hydrochloric acid, which breaks down food into absorbable components and neutralizes bacteria. Pepsinogen is converted to pepsin in this acidic environment, initiating protein digestion. The stomach gradually releases chyme into the small intestine.

Protein digestion continues in the small intestine with pancreatic enzymes including trypsin, chymotrypsin, carboxypeptidase, and elastase breaking proteins into peptides. Two brush-border peptidases—aminopeptidase and dipeptidase—complete this process by converting peptides into amino acids, which are then absorbed through the intestinal wall.

Bile, released from the liver into the small intestine, emulsifies fat by breaking it into smaller droplets. This increases the surface area for pancreatic lipase to act on triglycerides, breaking them into fatty acids and monoglycerides. These end products are then absorbed through the small intestinal wall.

Digestion begins in the mouth through mechanical breakdown by chewing and chemical action by salivary amylase, an enzyme in saliva that starts breaking down starches into maltose. This chewed food, called bolus, then travels down the esophagus to the stomach for continued digestion and ingestion and propulsion.

The small intestine absorbs monosaccharides, amino acids, fatty acids, water, electrolytes, vitamins, and minerals. The movement of villi allows these end products of digestion to come in contact with the absorbing membrane. This is the primary site where nutrient absorption occurs after enzymatic breakdown is complete.

Brush-border enzymes such as sucrase, lactase, and maltase break down disaccharides into monosaccharides in the small intestine. These enzymes are located on the intestinal lining and complete carbohydrate digestion that began with salivary amylase in the mouth and continued with pancreatic amylase in the small intestine.

Intrinsic factor, secreted by gastric glands in the stomach, combines with dietary vitamin B12 to prepare it for absorption in the ileum. This binding is essential for vitamin B12 to be properly absorbed later in the small intestine, ensuring adequate B12 levels for cellular function and metabolism.