Human Digestive System

The stomach produces proteases (pepsin) and gastric lipases, but it does not produce amylase. Salivary amylase is inactivated by the low pH of the stomach, and pancreatic amylase is produced by the pancreas, not the stomach.

Brush border enzymes are located on the microvilli of the small intestine. They perform the final stage of chemical digestion, breaking down small peptides and disaccharides into absorbable monomers.

Pancreatic lipase is the primary enzyme responsible for the chemical digestion of triglycerides into free fatty acids and monoglycerides. Bile salts emulsify fats, but they do not chemically digest them.

Cholecystokinin (CCK) is released by the duodenum in response to fats and proteins. It stimulates the pancreas to release digestive enzymes.

CCK causes the gallbladder to contract and the sphincter of Oddi to relax, which results in the release of bile into the duodenum to aid in fat digestion.

The intestinal phase occurs when chyme enters the duodenum. This triggers the enterogastric reflex, which inhibits gastric activity to allow the small intestine time to process the incoming chyme.

Parietal cells secrete intrinsic factor, which is essential for the absorption of vitamin B12 in the ileum. A lack of intrinsic factor leads to pernicious anemia.

Pepsinogen is an inactive zymogen. When it encounters the highly acidic environment created by hydrochloric acid (HCl) in the stomach, it is cleaved into its active form, pepsin.

Chief cells are responsible for the secretion of pepsinogen and gastric lipase into the stomach lumen.

Parietal cells are specialized cells in the gastric glands that secrete hydrochloric acid (HCl) and intrinsic factor.

The stomach is divided into the cardia, fundus, body, and pylorus. The pyloric region is the terminal part of the stomach that connects to the duodenum of the small intestine.

Peristalsis consists of waves of smooth muscle contraction that move the bolus or chyme forward through the digestive tract.

Deglutition (swallowing) has three phases: the buccal phase (voluntary), the pharyngeal phase (involuntary), and the esophageal phase (involuntary).

Salivary amylase is denatured by the low pH of the stomach. Therefore, it ceases to function shortly after the food enters the stomach.

Salivary amylase specifically targets the glycosidic bonds in starch, which is a carbohydrate polymer.