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Gastric Acid Secretion and Gastric Motility - Coggle Diagram
Gastric Acid Secretion and Gastric Motility
Histophysiology Of HCl Secretion (Carbonic Anhydrase & A-G-H Pathway)
CO₂ + Water → Carbonic acid → Hydrogen ions + Bicarbonate ions
Apical Membrane (Gastric Lumen)
Cl⁻ Channels → Passive Cl⁻ movement into the lumen.
H⁺/K⁺-ATPase (Proton Pump) → Actively secretes H⁺ into the lumen in exchange for K⁺
Basolateral Membrane (Blood)
Na⁺/K⁺-ATPase → Maintains Na⁺ and K⁺ electrochemical gradients.
Cl⁻/HCO₃⁻ Exchanger → Secretes HCO₃⁻ into the blood (Alkaline Tide).
The Three Major Stimulators (A–G–H Pathway)
Gastrin
Secreted by antral G cells; binds to CCK-B receptors on parietal cells and ECL cells.
Histamine
Released by Enterochromaffin-Like (ECL) cells → Binds to H₂ receptors on parietal cells → Strongly stimulates H⁺/K⁺-ATPase activity → Increases gastric acid secretion.
Acetylcholine (ACh)
Released by vagal efferents → Binds to M₃ receptors on parietal cells → Stimulates histamine release from ECL cells.
Regulation & Phases (Cephalic, Gastric, Intestinal & Feedback)
Gastric Phase (~60% of output; major phase)
Distension, peptides & amino acids
Local enteric reflexes & gastrin release
Somastostatin Feedback (Low pH trigger)
D cells release somatostatin
Inhibits G cells (Gastrin ↓)
Inhibits ECL cells (Histamine ↓)
Directly inhibits parietal cells
Cephalic Phase (~30% of output)
Sight, smell, thought
Vagus nerve (ACh / GRP)
Intestinal Phase (~10% of output)
Duodenal acid, fats, and hyperosmolarity
Secretin, CCK & GIP action
Gastric Motility & Emtyping (Pacemakers, Retropulsion & Kinetics)
Proximal Stomach (Fundus & Body)
Responsible for mixing and grinding
Powered by thick circular smooth muscle layers
Generates high-pressure antral contractions
Distal Stomach (Antrum & Pylorus)
Basal tone → 8–10 cmH₂O
Undergoes receptive relaxation
Acts as a food reservoir
Peristalsis, Retropulsion, and Grinding Mechanics
Retropulsion Cycle
1/ Peristaltic waves originate in the body and propagate toward the antrum.
2/ Small amounts of liquid chyme (particles < 1 mm) → Pass through the partially open pylorus → Enter the duodenum.
3/ The pylorus snaps shut before the antral wave finishes, forcing remaining chyme backward (retropulsion)
4/ Retropulsion grinds solid food and thoroughly mixes particles with gastric juices.
Pacemaker Cells
Located in the longitudinal muscle layer of the upper one-third of the gastric body; generate the basic electrical rhythm.
Modulation & Emptying Dynamics By Food Type
Accelerating Factors
Accelerates Emptying
Gastric distension → Activation of mechanoreceptors → Enteric ACh release → Increased antral contractions.
Slowing Factors
Delays / Slows Emptying
Duodenal inhibitory stimuli: Acid + Lipids + Hyperosmolarity + Particles > 1 mm + Caloric delivery > 3 kcal/min + Cold temperatures < 10°C + Stress (β-adrenergic sympathetic activation).
Liquids
Rapid & Exponential
Minimal resistance; flows quickly through the pylorus without requiring extensive grinding.
Digestible Solids
Initial lag phase → Followed by a linear phase:
Requires an initial antral grinding phase (retropulsion) before steady linear delivery into the duodenum.
Lipids & Fibers
Prolonged (4–5 Hours)
Triggers enterogastrones (CCK, secretin, GIP) via intestinal phase feedback, strongly inhibiting motor output.