Overview
This lecture explains the structure and main functions of the stomach within digestion, focusing on its mechanical role and chemical secretions, key cell types, regulation of gastric juices, and absorption processes.
Stomach Functions and Structure
- The stomach mainly mixes, churns, and pulverizes food, turning the food bolus into liquid chyme.
- Minimal nutrient absorption occurs in the stomach; its main role is mechanical digestion.
- Protein digestion begins in the stomach due to protein complexity; fat digestion starts but is minimal.
- The stomach contains regions such as the fundus, cardia, and has inner folds called rugae or gastric folds.
- Entry and exit are controlled by the cardiac sphincter (esophagus to stomach) and pyloric sphincter (stomach to small intestine).
Gastric Secretions and Cell Types
- Stomach lining contains goblet (mucous), parietal, and chief cells.
- Goblet cells secrete alkaline mucus to protect against stomach acid.
- Parietal cells produce hydrochloric acid, necessary to activate pepsinogen and aid vitamin B12 absorption.
- Chief cells secrete pepsinogen (inactive; becomes pepsin in acid to digest protein) and gastric lipase (begins fat breakdown).
Regulation of Gastric Secretions
- Secretion is controlled by nerves (parasympathetic system) and hormones.
- Gastrin (from pyloric region) increases gastric juices when food is present.
- There are three phases of secretion: cephalic (thinking about food, triggers secretions), gastric (food in stomach, stretch receptors trigger more secretions), and intestinal (chyme enters duodenum, secretions taper off).
Absorption in the Stomach
- Very limited absorption occurs; only alcohol, aspirin, water, glucose, and salts absorb here.
- Proteins, fats, and most carbohydrates are not absorbed in the stomach.
Regulation of Gastric Emptying
- Liquids leave the stomach quickly, solids more slowly; fats stay longest, causing greater satiety.
- Hormones like secretin and cholecystokinin (CCK) from the intestine help regulate gastric emptying and secretion.
- CCK slows gastric emptying if fat and protein are present, allowing more digestion time.
Key Terms & Definitions
- Bolus — Chewed ball of food entering the stomach.
- Chyme — Liquid mixture of food and gastric juices leaving the stomach.
- Rugae/Gastric Folds — Internal folds increasing stomach surface area for mixing.
- Goblet (Mucous) Cell — Secretes protective alkaline mucus.
- Parietal Cell — Produces hydrochloric acid for digestion and B12 absorption.
- Chief Cell — Secretes pepsinogen and gastric lipase for protein and fat digestion.
- Gastrin — Hormone that stimulates gastric juice secretion.
- Secretin — Hormone prompting pancreatic bicarbonate release to neutralize acid.
- Cholecystokinin (CCK) — Hormone slowing gastric emptying in response to fats/proteins.
Action Items / Next Steps
- Review major stomach cell types and their secretions.
- Study the three phases of gastric secretion.
- Remember which substances are absorbed in the stomach vs. small intestine.