Parietal cells secrets:
**Core Concept**
Parietal cells are a type of gastric mucosal cell located in the fundus of the stomach, playing a crucial role in acid-base homeostasis by secreting gastric acid (hydrochloric acid, HCl) and intrinsic factor. The secretion of gastric acid is essential for protein digestion and absorption of vitamin B12.
**Why the Correct Answer is Right**
Gastric acid secretion is mediated by the parietal cell's ability to secrete hydrogen and chloride ions. This process is facilitated by the proton pump (H+K+ ATPase) in the parietal cell membrane, which pumps hydrogen ions into the lumen. The chloride ion secretion is facilitated by the chloride-bicarbonate exchanger (AE2) in the basolateral membrane. The intrinsic factor, a glycoprotein, is secreted by the parietal cells and is essential for the absorption of vitamin B12 in the ileum.
**Why Each Wrong Option is Incorrect**
**Option A:** The correct answer is not secretin, as secretin is a hormone released by the duodenum in response to acidification, which stimulates pancreatic bicarbonate secretion and inhibits gastric acid secretion.
**Option B:** The correct answer is not gastrin, as gastrin is a hormone released by G cells in the antrum of the stomach, which stimulates gastric acid secretion by parietal cells but is not secreted by them.
**Option C:** The correct answer is not mucus, as mucus is secreted by mucous cells in the stomach, which protects the gastric mucosa from acid damage.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the proton pump inhibitor (PPI) class of medications, such as omeprazole, acts by irreversibly inhibiting the H+K+ ATPase in parietal cells, thereby reducing gastric acid secretion.
**Correct Answer: D. Gastric acid and intrinsic factor**