Overview
Concise notes on the digestive processes of the stomach, focusing on anatomy, phases of gastric secretion, and hormonal regulation.
Anatomy of the Stomach
- Stomach regions include cardia, fundus, body, and pylorus; each supports digestion roles.
- Rugae allow expansion after food intake; increase surface area for secretion and mixing.
- Gastric pits lead to gastric glands containing specialized secretory cells.
- Mucosa secretes mucus and bicarbonate forming a protective barrier against acid.
- Muscularis externa has three layers; oblique, circular, longitudinal for churning.
- Pyloric sphincter regulates chyme release into the duodenum.
Gastric Secretion: Regulation of Gastric Activity
- Gastric activity is regulated by neural and hormonal mechanisms across three phases.
- Vagus nerve stimulates secretions and motility; enteric reflexes coordinate local control.
- Feedback from the intestine modifies gastric secretion based on chyme properties.
- Hormones modulate acid, enzyme release, and motility to optimize digestion.
Cephalic Phase
- Initiated by sight, smell, taste, or thought of food; before food enters stomach.
- Parasympathetic impulses via vagus nerve stimulate gastric glands and motility.
- Increases secretion of acid, pepsinogen, and mucus preparing the stomach.
Gastric Phase
- Begins when food reaches the stomach; distension and peptides trigger responses.
- Stretch receptors and chemoreceptors activate local and vagovagal reflexes.
- Gastrin release increases HCl secretion, pepsinogen activation, and mixing.
- Adequate pH maintained; excessive acidity reduces gastrin to prevent over-secretion.
Intestinal Phase
- Starts as chyme enters the duodenum; balances gastric emptying with intestinal capacity.
- Brief stimulatory effect via intestinal gastrin may occur early.
- Predominantly inhibitory: enterogastric reflex reduces gastric motility and secretion.
- Presence of acid, fats, and hypertonic chyme in duodenum strengthens inhibition.
Hormonal Regulation
- Gastrin from G cells increases HCl secretion and gastric motility.
- Secretin reduces gastric secretion; stimulates bicarbonate from pancreas to neutralize acid.
- Cholecystokinin (CCK) slows gastric emptying; stimulates gallbladder and pancreatic enzymes.
- Gastric inhibitory peptide (GIP) decreases gastric activity; modulates insulin release.
Key Terms & Definitions
- Gastrin: Hormone that increases gastric acid secretion and motility.
- Enterogastric reflex: Duodenal reflex inhibiting gastric secretion and emptying.
- Chyme: Semi-fluid mixture of partially digested food and gastric secretions.
- Rugae: Folds in stomach lining that allow expansion and increase surface area.
- Pyloric sphincter: Valve controlling chyme passage from stomach to duodenum.
Action Items / Next Steps
- Review the sequence and controls of cephalic, gastric, and intestinal phases.
- Memorize key hormones (gastrin, secretin, CCK, GIP) and their primary actions.
- Practice tracing neural pathways: vagus nerve roles and enteric reflexes.