Digestive Physiology: Stomach, Digestion, and Regulation

Energy-forming nutrients

Carbohydrates, fats, and proteins; must be broken down into absorbable units in the GI tract.

Monosaccharides

The absorbable form of carbohydrates (e.g., glucose, fructose, galactose).

Glycerol and free fatty acids

The absorbable forms of fats after digestion.

Small peptides or amino acids

The absorbable forms of proteins after digestion.

Kreb's cycle (TCA)

Where the pathways for carbohydrates, fats, and proteins converge to produce ATP.

Chief cells

Gastric cells that produce pepsinogen.

Parietal cells

Gastric cells that produce hydrochloric acid ($HCl$) via the proton pump and intrinsic factor.

Pepsinogen

An inactive enzyme produced by chief cells, activated to pepsin by $HCl$ in the stomach lumen to begin protein digestion.

Proton pump (H+/K+ ATPase)

Located in parietal cells, it pumps hydrogen ions into the gastric lumen to form $HCl$.

Three keys to start the proton pump

Gastrin, histamine, and acetylcholine; all three are required to activate the parietal cell proton pump for $HCl$ production.

Gastrin

A hormone released from G cells in the antrum that stimulates parietal cells and relaxes the fundus.

Histamine (in GI context)

Released by mast cells in the GI mucosa; acts on H2 receptors on parietal cells to promote acid secretion.

Acetylcholine (in GI context)

A parasympathetic neurotransmitter that stimulates acid secretion from parietal cells during rest-and-digest states.

Secretin

An intestinal hormone released by small intestinal epithelial cells in response to low duodenal pH; stimulates pancreatic bicarbonate release and slows gastric motility.

CCK (Cholecystokinin)

An intestinal hormone released in response to fats and proteins in the duodenum; stimulates gallbladder contraction, pancreatic enzyme release (lipases, proteases), and slows gastric motility.

Enterogastric reflex

A feedback mechanism where the duodenum signals the stomach to regulate gastric emptying based on pH and chyme content.

Prostaglandins (in gastric protection)

Lipid compounds produced by epithelial cells that support mucosal blood flow and stimulate bicarbonate production, helping maintain barrier integrity and mucosal health.

NSAIDs (Nonsteroidal Anti-inflammatory Drugs)

Medications that can block protective prostaglandins, increasing the risk of gastric ulcers and kidney issues.

Intrinsic factor

Produced by parietal cells, essential for vitamin B12 absorption; deficiency can lead to B12 malabsorption.