How Parietal Cells Aid Digestion and Stomach Acid Secretion
Parietal cells are one type of special cell that play a very crucial part in the digestive system of humans. Parietal cells are also called oxyntic cells. Parietal cells are one type of secretory epithelium cells that secrete hydrochloric acid. This hydrochloric acid makes the gastric juice acidic (pH 2.0 - 3.7). These oxyntic cells are found in the inner linings of the stomach. Besides hydrochloric acid, parietal cells secrete an intrinsic factor that helps in the absorption of vitamin B12. Such secretion of oxyntic cells is controlled by central and local modulators by some regulating factors such as histamine, acetylcholine, gastrin, etc.
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Structure and Location of Parietal Cells
Parietal cells are part of the human stomach. The stomach is the widest and distensible C-shaped part of the alimentary canal which is obliquely behind the diaphragm on the left side. The stomach wall is made of many folds that are known as gastric rogues. The stomach has mainly three parts - fundus, body and pyloric. Gastric parietal cells are the glandular part of the fundus and body regions of the stomach.
To explain the question of what are parietal cells, it is necessary to mention that it is a kind of epithelium cell that secrete hydrochloric acid and intrinsic factors. Oxyntic cells are made of canaliculi. Canaliculi is an extensive secretory network. To increase the surface area such canaliculi has a deep infolding. An interesting mechanism is followed here. Interestingly the number of canaliculi largely depends on the secretory needs. This canaliculi network helps in the secretion of hydrochloric acid. In the stomach, the secretion of hydrochloric acid needs active transport as it is transported against the H+concentration. That is why the enzyme hydrogen potassium ATPase is present in parietal cells to conduct active transport of hydrochloric acid.
Function of Oxyntic Cells
To discuss parietal cells function, it is needed to mention that to work properly gastric digestive enzymes require an acidic medium. Hydrochloric acid secreted by parietal cells maintains the acidic medium within the stomach.
The source of the hydrogen ion of hydrochloric acid is carbonic acid which is formed by carbon dioxide and water with the help of an enzyme called carbonic anhydrase. There is an exchange of bicarbonate ions with chloride ions through the membrane of parietal cells. Such ion exchange results in an alkaline tide phenomenon. Huge amounts of potassium and chloride ions are absorbed by simple diffusion. With the help of a potassium hydrogen pump and ATP, the hydrogen ion is absorbed into the parietal cells by the active ion exchange method. Interestingly the efficiency of this hydrogen potassium pump increases during the activation of parietal cells stomach.
Hydrochloric acid plays a vital role in the digestion of food within the stomach. It helps in the digestion of proteins by unfolding the peptide bonds of protein and thus helps the gastric enzymes to work properly on it. Hydrochloric acid transforms the inactive pepsinogen and prorenin into active enzymes pepsin and renin and thus helps in proteolysis. Parietal cells secrete an intrinsic factor which is one type of glycoprotein. This intrinsic factor plays an important role in the absorption of vitamin B12.
Regulation of Parietal Cells
Secretion of oxyntic cells is controlled by central and local modulators by some regulating factors such as histamine, acetylcholine, gastrin, etc. Histamine plays a major role in such stimulation by activating H₂ histamine receptors. On the other hand, the vagus nerve and enteric nervous system control the secretion by activating the C3 receptors with the help of acetylcholine. Gastrin activates the CCK2 receptors and also contributes to the secretion of histamine.
Did You Know?
Parietal cells secrete an intrinsic factor which is one type of glycoprotein. This intrinsic factor plays an important role in the absorption of vitamin B12. The deficiency of vitamin B12 can cause megaloblastic anaemia which can affect the transport of oxygen in the blood. Autoimmune destruction of parietal cells causes pernicious anaemia. Thus, the intrinsic factor secreted by oxyntic cells plays an important role in the formation of haemoglobin.
Intracellular cAMP is stimulated by H2 receptors of parietal cells and the calcium level is increased by M3 and CCK2 receptors. That results in the increased activity of kinase A. Kinase A induces the cell membrane and increases the hydrogen ion secretion and the potassium ion reabsorption and that makes the secretion of hydrochloric acid within the stomach cavity.
FAQs on Parietal Cell: Definition, Structure, and Functions
1. What are parietal cells and where are they found in the human body?
Parietal cells, also known as oxyntic cells, are specialised cells found in the lining of the stomach. They are located within the gastric glands, primarily in the upper and middle parts of the stomach (the fundus and body). Their main job is to assist in the digestive process.
2. What are the two main substances secreted by parietal cells?
Parietal cells are responsible for secreting two crucial substances into the stomach:
- Hydrochloric acid (HCl): This creates a highly acidic environment in the stomach, which is necessary to kill germs and break down food.
- Intrinsic factor: This is a special protein (glycoprotein) that is essential for your body to absorb vitamin B12 in the small intestine.
3. Why is the hydrochloric acid produced by parietal cells so important for digestion?
The hydrochloric acid (HCl) produced by parietal cells is vital for several reasons. It sterilises the food you eat by killing harmful bacteria and pathogens. It also helps to unravel proteins, making them easier to digest. Most importantly, HCl activates an enzyme called pepsinogen, turning it into its active form, pepsin, which is the main enzyme for digesting proteins in the stomach.
4. How do parietal cells and chief cells work together as a team in the stomach?
Parietal cells and chief cells have a partnership essential for protein digestion. Chief cells secrete an inactive enzyme called pepsinogen. This enzyme cannot digest anything on its own. The hydrochloric acid (HCl) secreted by the parietal cells creates the acidic conditions needed to activate pepsinogen and convert it into pepsin. Once active, pepsin can effectively break down proteins in food.
5. What would happen to our body if parietal cells stopped functioning correctly?
If parietal cells were damaged or failed to work, it would cause significant health problems. Without hydrochloric acid, protein digestion would be inefficient, and the risk of gastrointestinal infections would increase. More critically, the lack of intrinsic factor would prevent the absorption of vitamin B12, leading to a serious condition known as pernicious anaemia, which affects the body's ability to make healthy red blood cells.
6. How do parietal cells manage to release such a large amount of acid so quickly?
Parietal cells have an amazing ability to change their structure. They contain a network of tiny internal channels called canaliculi. When the cell is resting, this network is small. But when stimulated by the presence of food, these channels expand and unfold, dramatically increasing the cell's surface area. This allows for the rapid and efficient secretion of hydrochloric acid directly into the stomach.
7. What controls the acid secretion from parietal cells?
The secretion of acid by parietal cells is not constant; it's carefully regulated by the body's nervous and endocrine systems. Three main chemical messengers stimulate acid release:
- Acetylcholine: A neurotransmitter from nerve endings.
- Gastrin: A hormone released when food stretches the stomach.
- Histamine: A chemical released by nearby cells in the stomach lining.
These signals ensure that acid is only produced when it is needed for digestion.
