🍽️ Gastrointestinal Physiology: Digestion & Secretion
The gastrointestinal tract integrates cephalic, gastric, and intestinal regulatory phases to execute nutrient enzymatic digestion, mucosal transport, and fluid-electrolyte reclamation.
1. Parietal Cell Gastric Acid (HCl) Secretion Mechanism
Gastric parietal cells secrete hydrochloric acid ($pH approx 0.8–1.5$) to denature proteins and activate pepsinogen:
- Carbonic anhydrase (CA-II) converts $CO_2 + H_2O rightarrow H^+ + HCO_3^-$.
- Apical $H^+/K^+$ ATPase pump actively pumps $H^+$ into the gastric lumen in exchange for $K^+$, against a million-fold concentration gradient. Target of proton pump inhibitors (omeprazole).
- Basolateral $HCO_3^-/Cl^-$ anion exchanger exports bicarbonate into blood (creating postprandial alkaline tide) and imports $Cl^-$, which exits into the lumen via apical CFTR/ClC channels.
- Secretagogues: Gastrin (via $CCK_B$), Histamine (via $H_2$ receptors / cAMP), and Acetylcholine (via $M_3$ / $Ca^{2+}$) stimulate secretion; Somatostatin and Prostaglandins inhibit.