Parietal cells, also known as oxyntic cells, are specialized cells located within the lining of the stomach. These cells are primarily found in the gastric glands, specifically in the fundus and body regions of the stomach. They play a fundamental role in the digestive process by producing and secreting substances essential for breaking down food and protecting the body.
Key Secretions of Parietal Cells
Parietal cells are responsible for secreting two primary substances: hydrochloric acid (HCl) and intrinsic factor. Hydrochloric acid is a strong acid, creating an acidic environment within the stomach with a pH ranging between 1.5 and 3.5.
The other secretion is intrinsic factor, a glycoprotein. Unlike hydrochloric acid, intrinsic factor does not directly participate in the chemical breakdown of food within the stomach. Its role becomes important later in the digestive tract, particularly concerning nutrient absorption.
Roles of Parietal Cell Secretions
The hydrochloric acid secreted by parietal cells performs several important functions in digestion. It helps denature proteins in ingested food, exposing their peptide bonds for subsequent enzymatic breakdown. This acidic environment also activates pepsinogen into its active form, pepsin, an enzyme that initiates protein digestion. Beyond digestion, the strong acidity serves as a defense mechanism, killing many harmful bacteria and other microorganisms that may enter the body with food. Gastric acid also aids in the absorption of certain minerals, including iron, calcium, and phosphate.
Intrinsic factor plays an important role by enabling the absorption of vitamin B12. After vitamin B12 is released from food in the stomach, it binds to intrinsic factor. This complex then travels to the ileum, where it is absorbed into the bloodstream. Without intrinsic factor, dietary vitamin B12 cannot be absorbed.
How Parietal Cell Activity is Controlled
The activity of parietal cells is regulated by signals that ensure appropriate acid secretion. Three main stimulators prompt these cells to increase their output. Acetylcholine, a neurotransmitter released from the vagus nerve, directly stimulates parietal cells. Gastrin, a hormone produced by G cells in the stomach, also acts as a stimulant by triggering histamine release.
Histamine, released from enterochromaffin-like (ECL) cells, binds to receptors on parietal cells, leading to increased acid secretion. This ensures that acid production ramps up when food enters the stomach. Conversely, somatostatin, a hormone released by D cells, acts as an inhibitor, reducing the secretion of acid by directly affecting parietal cells and by inhibiting gastrin and histamine release. The presence of acid in the duodenum also signals a reduction in gastric acid secretion, completing a feedback loop.
Consequences of Parietal Cell Dysfunction
When parietal cells do not function correctly, either producing too much or too little acid, various health issues can arise. Excessive stomach acid secretion, known as hyperchlorhydria, can overwhelm the stomach’s protective mechanisms, leading to conditions like peptic ulcers. Gastroesophageal reflux disease (GERD) is another common problem associated with high acid levels, causing discomfort and potential damage to the esophageal lining.
Conversely, insufficient acid secretion also poses significant challenges. Low stomach acid can impair the digestion of proteins, as pepsinogen may not be activated. This can lead to nutrient deficiencies, including malabsorption of minerals like iron and calcium, and notably, vitamin B12. Without enough intrinsic factor, vitamin B12 cannot be absorbed, leading to a condition called pernicious anemia. Low stomach acid also reduces the body’s defense against ingested pathogens, potentially increasing susceptibility to bacterial overgrowth in the digestive tract.