Final answer:
Parietal cells secrete hydrochloric acid (HCl) and intrinsic factor, thereby enabling digestion and absorption of nutrients. They contribute to the highly acidic environment of the stomach, critical for the activation of the enzyme pepsin. The stomach's acidity is regulated to prevent excess and damage, and mechanisms are in place to ensure the proper digestion of proteins and absorption of vitamin B12.
Step-by-step explanation:
Parietal cells, located in the middle region of the gastric glands, play a crucial role in digestion by secreting hydrochloric acid (HCl) and intrinsic factor into the stomach. The release of HCl by these cells is what creates the highly acidic environment in the stomach, with a pH ranging from 1.5 to 3.5, which is needed to activate pepsin from its inactive form, pepsinogen. Pepsin is the enzyme responsible for the digestion of proteins. Moreover, the low pH denatures proteins and helps to kill ingested bacteria. The intrinsic factor released by parietal cells is vital for vitamin B12 absorption in the small intestine. The acidity in the stomach is regulated by various mechanisms, including feedback loops that modulate HCl secretion to prevent excessive acidification which can lead to conditions like heartburn.
During the gastric phase of secretion, which lasts 3 to 4 hours, the distention of the stomach from ingested food activates stretch receptors. These in turn trigger the release of acetylcholine and gastrin, further stimulating the secretion of gastric juice and HCl from parietal cells. The production of HCl is crucial to maintain the acidic pH necessary for proper digestion of proteins. To protect the stomach lining from the acidic environment, mucus is secreted alongside HCl.
When the partially digested food (now termed chyme) moves from the stomach to the small intestine, the hormone secretin prompts the pancreas to release bicarbonate (HCO3), which neutralizes the acid, raising the pH to around 7.0. This change in pH helps to prepare for further digestion and absorption in the small intestine, which is facilitated by a variety of pancreatic enzymes.