48 - Secretion, enzymatic breakdown and absorption in the stomach. Gastric juice: composition, secretion and functions. Regulation of gastric secretion- cephalic, gastric and intestinal phases. Protective properties of the gastric barrier.

sections

secretion and enzymatic breakdown

effect of gastrin

secretion and regulation

gastric mucosal barrier

secretion and enzymatic breakdown

• The stomach produces 2–3 L of gastric juice daily, containing water, HCl, mucus, and enzymes.

• Gastric juice mixes with food to aid digestion.

• Mucous cells secrete mucus to protect the stomach lining from acid and enzymes.

• Parietal cells secrete HCl (kills bacteria) and intrinsic factor (needed for vitamin B12 absorption).

• Chief cells secrete pepsinogen (inactive form of pepsin) and gastric lipase.

  • Pepsin (activated by HCl) breaks down proteins.

  • Gastric lipase helps digest fats.

• Pepsinogen is secreted in inactive form to prevent damage to chief cells.

• Release of hormone gastrin into the bloodstream in response to many stimuli, such as signals from the vagus nerve

  • Glandular stimulation by gastrin leads to increased secretion of gastric juice to increase digestion.

  • Stimulation of smooth muscles by gastrin leads to stronger contractions of the stomach and the opening of the pyloric sphincter to move food into the duodenum.

  • Gastrin binds to receptor cells in the pancreas and gallbladder →increases the secretion of pancreatic juice and bile.

effect of gastrin

• Release of hormone gastrin into the bloodstream in response to many stimuli, such as signals from the vagus nerve

  • Glandular stimulation by gastrin leads to increased secretion of gastric juice to increase digestion.

  • Stimulation of smooth muscles by gastrin leads to stronger contractions of the stomach and the opening of the pyloric sphincter to move food into the duodenum.

  • Gastrin binds to receptor cells in the pancreas and gallbladder →increases the secretion of pancreatic juice and bile.

secretion and regulation

• WATER is absorbed by the stomach, drugs eg, aspirin, about 20% of alocohol, some amino acids

• Phases of Gastric Acid Secretion

  1. Cephalic Phase (≈30%)

     • Triggered by the sight, smell, taste, or thought of food (anticipation of eating).

     • Mediated by the vagus nerve (parasympathetic response).

     • Vagal stimulation activates:

       • Parietal cells → secrete HCl

       • Enterochromaffin-like (ECL) cells → release histamine

       • G cells → stimulated by gastrin-releasing peptide (GRP) → release gastrin, which further stimulates acid secretion.

     • The vagus nerve also inhibits somatostatin release, removing its inhibitory effect on acid production.

  2. Gastric Phase (≈50%)

     • Initiated by food entering the stomach.

     • Stimuli include:

       • Stomach distension (activates stretch receptors and mechanoreceptors)

       • Presence of peptides and amino acids in the stomach contents.

     • These factors further stimulate G cells, ECL cells, and parietal cells, enhancing acid secretion.

  3. Intestinal Phase (≈10%)

     • Begins when chyme enters the small intestine.

     • Triggered by intestinal distension and nutrients (especially amino acids) in the chyme.

     • Provides a minor stimulation of gastric acid production compared to earlier phases.

  Regulation of Gastric Acid Secretion

  • Stimulated by:

    • Parasympathetic nervous system (via the vagus nerve)

    • Gastrin (from G cells)

    • Histamine (from ECL cells)

  • Inhibited by:

    • Somatostatin (from D cells)

    • VIP (vasoactive intestinal peptide)

    • CCK (cholecystokinin)

    • Secretin

gastric mucosal barrier

• Barrier has 3 protective components which provide resistance for mucosal surface of the stomach

  1. Bicarbonate ions, secreted by the surface epithelial cells. -neutralize acids.

  2. compact epithelial cell lining

  3. insoluble mucous covering- protective

• Factors that can damage the barrier:

  • Bacterial Infection by Helicobacter pylori

  • Alcohol

  • NSAID

  • ASPIRIN