Parietal cells are specialized, acid-secreting cells of the stomach. Their primary function is to produce hydrochloric acid; a fundamental component of gastric fluid that plays key roles in protein digestion and the destruction of ingested bacteria.
What is a Parietal Cell?
Parietal cells (AKA oxyntic cells) are specialized, acid-secreting cells found in the stomach. The main function of parietal cells is to produce hydrochloric acid (HCl), which is responsible for maintaining a low pH environment in the stomach.
Location of Parietal Cells
Parietal cells are found in gastric glands located in the lining of the fundus and body regions of the stomach. The fundus is the upper, curved part of the stomach that bulges out near the esophagus, and the body is the main part of the stomach.
Function of 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 primary function of a parietal cell is to produce hydrochloric acid, which is an essential component of gastric fluid.
Gastric fluid (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 value of 1.5 – 2.0, and its main function is to kill microorganisms that are ingested with food. This prevents potentially harmful bacteria from reaching the intestines.
Stomach acid also creates a low pH environment, which is essential for proper protein digestion. Pepsin is a protein-digesting enzyme that is produced by cells in the stomach lining and activated by a pH of 1.5 – 2.0. Once activated, pepsin breaks proteins down into smaller peptides and amino acids, which can then be absorbed in the small intestine.
Intrinsic Factor Production
Parietal cells also secrete a glycoprotein called the intrinsic factor (IF). IF is crucial for the transportation and absorption of vitamin B12 in the final section of the small intestine (the ileum). Insufficient levels of IF in the stomach can lead to a variety of B12 deficiency-related conditions, including anemia and neurological disorders.
Stimulation of Parietal Cells
Parietal cells are stimulated to secrete HCl and IF during three phases of digestion; the cephalic phase, the gastric phase, and the intestinal phase.
The Cephalic Phase
During the cephalic phase of digestion, acid and IF secretion begins before food even enters the stomach. The sight, smell, taste, or thought of food stimulates the vagus nerve, which transmits electrical impulses to parietal cells in the stomach and initiates acid secretion.
The cephalic secretion phase typically accounts for about 20% of total acid secretion that occurs when eating a meal.
The Gastric Phase
The gastric phase of digestion begins after food is swallowed. This phase accounts for around two-thirds of total acid secretion. When food reaches the stomach, it stimulates the parietal cells in two ways; by increasing the pH of the stomach contents, and by stretching the gastric walls. This initiates a cascade of events that ultimately leads to the release of hydrochloric acid from the parietal cells. The increased acid production lowers the pH in the stomach to 1.5 – 2.0, creating 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 contents of the stomach move into the upper part (or duodenum) of the small intestine. Initially, acid secretion in the stomach continues, but the mixture of acid and semi-digested fats in the intestine soon triggers the enterogastric reflex. When this happens, the duodenum sends inhibitory signals to the stomach and gastric acid secretion returns to its basal level.
Mechanism of Hydrochloric Acid Production
Acid secretion takes place in the canaliculi of parietal cells. The canaliculi are deep infoldings in parietal cells which increase their surface area for acid production.
During acid production, carbon dioxide combines with water in the cytoplasm of parietal cells to produce carbonic acid (H2CO3). Carbonic acid is an unstable compound that quickly splits into a hydrogen ion (H+), and a bicarbonate ion (HCO3-). Next, the hydrogen ions are actively pumped into the canaliculi in exchange for potassium ions via the H+/K+ ATPase ion pump. The bicarbonate ion is transported out of the parietal cell and 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.
Adaptations of Parietal Cells
Parietal cells are specialized epithelial cells and are highly adapted for hydrochloric acid production. The cytoplasm of parietal cells contains abundant mitochondria, which produce the ATP required for the active transport of ions in and out of the cell. Parietal cells also have highly specialized structures called canaliculi. These are deep infoldings in the plasma membrane of the cell that are covered in microvilli and greatly increase the surface area for acid secretion.
Another feature found in all parietal cells are the tubulovesicles. 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.