Parietal cells are specialized, acid-secreting cells in the stomach. Their main function is to produce hydrochloric acid; a fundamental component of gastric fluid that plays a key role in protein digestion and the destruction of ingested bacteria.
What is a parietal cell?
Parietal cells (AKA oxyntic cells) are specialized, acid-secreting cells in the stomach. The main function of the parietal cells is to produce hydrochloric acid (HCl), which is responsible for maintaining a low pH level in the stomach.
Parietal cell location
Parietal cells are located in gastric glands, which are located in the lining of the stomach Fundus and body Regions of the stomach. The fundus is the upper, curved part of the stomach that bulges near the esophagus, and the body is the main part of the stomach.
Function of the parietal cells
Parietal cells produce two substances that are essential for proper digestion and nutrient absorption; Hydrochloric acid and intrinsic factor.
Hydrochloric acid production
The main function of a parietal cell is to produce hydrochloric acid, which is an integral part of gastric fluid.
Gastric juice (AKA Stomach acid) has several digestive functions. It is an acidic mixture of hydrochloric acid, water, electrolytes and the enzyme pepsin. Stomach acid typically has a pH of 1.5-2.0 and its main function is to kill microorganisms ingested with food. This prevents potentially harmful bacteria from entering the intestines.
Stomach acid also creates a low pH environment that is essential for proper protein digestion. Pepsin is a protein-digesting enzyme that is produced by cells in the gastric mucosa and activated by a pH value of 1.5 – 2.0. Once activated, pepsin breaks down proteins into smaller peptides and amino acids, which can then be taken up in the small intestine.
Intrinsic factor production
Parietal cells also secrete a glycoprotein called intrinsic factor (IF). IF is critical to the transport and absorption of vitamin B12 in the last section of the small intestine (the Ileum). Inadequate levels of IF in the stomach can lead to a variety of B12 deficiency conditions, including anemia and neurological disorders.
Parietal cell stimulation
Parietal cells are stimulated to secrete HCl and IF during three phases of digestion; the head phase, the stomach phase and the intestinal phase.
The cephalic phase
During the cephalic digestive phase, acid and IF secretion begin before food even reaches the stomach. The sight, smell, taste or thought of food stimulates them Vagus nerve, which transmits electrical impulses to the parietal cells in the stomach and sets the acid secretion in motion.
The cephalic secretion phase typically accounts for about 20% of the total acid secretion that occurs when eating a meal.
The stomach phase
The gastric phase of digestion begins after the food is swallowed. This phase accounts for about two thirds of the total acid secretion. When the food reaches the stomach, it stimulates the parietal cells in two ways; by increasing the pH of the stomach contents and by stretching the stomach walls. This sets in motion a cascade of events that ultimately lead to the release of hydrochloric acid from the parietal cells. The increased acid production lowers the pH value in the stomach to 1.5 – 2.0 and thus creates optimal conditions for protein digestion.
The intestinal phase
5 – 10% of acid secretion takes place during the intestinal phase of digestion. This phase begins when the stomach contents reach the upper part (or Duodenum) of the small intestine. Initially, acid secretion continues in the stomach, but the mixture of acid and half-digested fats in the intestine soon triggers the enterogastric reflex. In this case, the duodenum sends inhibitory signals to the stomach and gastric acid secretion returns to its basic level.
Mechanism of Hydrochloric Acid Production
The acid secretion takes place in the Canaliculi of parietal cells. The canaliculi are deep folds in parietal cells that increase their surface area for acid production.
During acid production, carbon dioxide combines with water in the cytoplasm of the parietal cells carbonic acid (H2CO3). Carbonic acid is an unstable compound that quickly turns into a. splits Hydrogen ion (H +) and a Bicarbonate ion (HCO3-). The hydrogen ions are then actively pumped into the canaliculi in exchange for potassium ions via the H + / K + ATPase ion pump. The bicarbonate ion is transported from the parietal cell into the blood in exchange for Chloride ions (Cl-), which are then transported into the canaliculi via a chloride channel. Finally, the hydrogen ions and chloride ions combine to form hydrochloric acid.
Adjustments of parietal cells
Parietal cells are specialized epithelial cells and are highly adapted to hydrochloric acid production. The parietal cell cytoplasm contains abundant mitochondria, which produce the ATP needed to actively transport ions in and out of the cell. Parietal cells also have highly specialized structures called canaliculi. These are deep folds in the plasma membrane of the cell that are covered with microvilli and greatly increase the surface area for acid secretion.
Another characteristic that is found in all parietal cells is the tubulovesicle. These are networks of tubules and vesicles that contain the H + / K + ATPase ion pump and that fuse with the canalicular membrane during acid secretion.