Explanation:
In parietal cells, CO₂ combines with water to form carbonic acid (H₂CO₃) through the action of carbonic anhydrase. This carbonic acid dissociates into H⁺ and HCO₃⁻, providing hydrogen ions for HCl secretion. :contentReference[oaicite:0]{index=0}

Explanation:
Hydrogen ions produced in parietal cells are actively pumped into the gastric lumen by the K⁺/H⁺ ATPase (proton pump). This is the final step in gastric acid secretion.

Explanation:
During active HCl secretion, bicarbonate ions are released into the blood in exchange for chloride entering parietal cells, temporarily increasing blood pH. This is called the alkaline tide.

Explanation:
Chief cells secrete pepsinogen, the inactive precursor of pepsin. It requires an acidic environment to be converted into the active enzyme pepsin.

Explanation:
Pepsin functions best in very acidic conditions, with an optimal pH between 1 and 2 in the stomach.

Explanation:
G cells located in the gastric antrum secrete gastrin, which stimulates acid secretion.

Explanation:
Histamine acts on H2 receptors on parietal cells, activating adenyl cyclase and increasing gastric acid secretion.

Explanation:
D cells release somatostatin when gastric pH drops below 2. Somatostatin inhibits gastric acid secretion.

Explanation:
Oxyntic glands make up about 80% of gastric glands and are located in the body and fundus of the stomach.

Explanation:
The stomach secretes approximately 2 liters of gastric juice per day.

Explanation:
The cephalic phase occurs before food enters the stomach and accounts for around 30% of gastric secretion.

Explanation:
The gastric phase is the most significant phase and accounts for more than 60% of gastric secretion.

Explanation:
Acetylcholine released from vagal stimulation activates parietal and chief cells during the cephalic phase.

Explanation:
Peptides present in the stomach strongly stimulate gastrin release from G cells.

Explanation:
Gastrin from the intestine initially stimulates gastric secretion before inhibitory hormones dominate.

Explanation:
Gastric inhibitory polypeptide (GIP) inhibits gastric secretion and stimulates insulin secretion.

Explanation:
The mucus layer contains high concentrations of bicarbonate which help neutralize acid and protect the mucosa.

Explanation:
Prostaglandins increase mucus production and mucosal blood flow, protecting the gastric epithelium.

Explanation:
NSAIDs inhibit cyclooxygenase-1 (COX-1), decreasing prostaglandin production and weakening gastric mucosal protection.

Explanation:
Brunner’s glands are located in the duodenum and secrete alkaline mucus to protect the mucosa.

Explanation:
Brunner’s glands secrete alkaline mucus with a pH of about 8–8.9.

Explanation:
Crypts of Lieberkühn contain goblet cells that secrete mucus and enterocytes that secrete watery alkaline fluid.

Explanation:
Enterocytes secrete a watery alkaline fluid similar to extracellular fluid which facilitates absorption.

Explanation:
Water moves across intestinal epithelium mainly by osmosis following electrolyte movement.

Explanation:
Local reflexes from the duodenum inhibit gastric secretion; this is termed the reverse enterogastric reflex.

Explanation:
Highly acidic chyme entering the duodenum stimulates inhibitory mechanisms that suppress gastric secretion.

Explanation:
H2 receptor antagonists such as cimetidine block histamine H2 receptors on parietal cells, reducing acid secretion.

Explanation:
Proton pump inhibitors block the K⁺/H⁺ ATPase pump in parietal cells, preventing hydrogen ion secretion.

Explanation:
HCl converts pepsinogen into active pepsin by hydrolytic cleavage.

Explanation:
Once pepsin is formed, it can activate remaining pepsinogen molecules through an autocatalytic mechanism.