Gastric acid secretion physiology
The mechanism by which parietal cells transport hydrogen ions against concentration gradient is not fully understood. It is believed that the ionization of water is the main source of the production of hydrogen ions (H+). When water becomes ionized, one molecule of H+ and H- when it is given external energy. External supply of energy may be in the form of ultraviolet light or as in living systems oxidative phosphorylative processes which is coupled with cytochrome system. The H+ ions produced from this unnatural process is actively secreted into the lumen of the gut. Chloride ions (C1-) on the other hand are actively transported from the interstitial fluid into the lumen. The H+ and C1- ions are secreted initially into the intracellular canaliculi before they are transported to the lumen. H+
H20 H+ & 0H+
May also be generated from other systems within the cell. For example, substitutes like glucose via the flavoprotein – cytochrome respiratory enzyme chain. For each H+ secreted into the lumen, one molecule of 0H- is kept within the cell.
Bicarbonates HC03 are extremely important in production of gastric acid by parietal cells. Carbonic anhydrase enzyme has been shown to be in abundance within the gastric lumen. Thus the gastric arterial circulation brings in carbondioxide (C02) which is extracted by parietal cells and with the addition of water under the influence of carbonic anhydrase leads to the secretion of carbonic acid in the parietal cell.
H20 + C02 = H2C03 Carbonic anhydrase
This carbonic acid (H2C03) then breaks down into HC03 and H+ ions.
H2C03 HC03- + H+
The bicarbonate ion thus formed goes back into the circulation, so that the venous return from the stomach is usually alkaline and contains a high level of HC03- but with a reduced C02 concentration.
H+ ion derived from the breakdown of H2C03 is coupled with the 0H- which is kept in the cell in order to neutralize the alkalinity of the cell which is caused by accumulation of 0H-.
0H- + H+ = H20
The process of Na+ and K+ exchange through the Na+ pump continues unabated. The energy for the active transport of hydrogen ions across parietal cell membrane comes from aerobic glyucolysis. Acid secretion is depressed by anaerobic conditions and inhibitors of oxidative phosphorylation like dinitropherol causes the reduction in acid elaboration.
Gastric acid analysis are performed by the following indications
To determine the ability of the patient to secret HCL as in achlorhydria
To determine the amount of acid secreted by the individual
To determine the type of surgery to be performed because in incomplete vagotomy for peptic ulcer, stromal ulcer may follow the ordinary procedure.
The following are performed in gastric acid analyses.
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Creator: Oluwole Ogunranti