Physiology
Gastrointestinal
Secretion of hydrochloric acid from parietal cells is stimulated by all of the following EXCEPT for:
Answer:
Secretion of hydrochloric acid from parietal cells is stimulated by vagal stimulation (via the neurotransmitters acetylcholine and gastrin-releasing peptide), histamine and gastrin. Somatostatin inhibits the release of hydrochloric acid.Gastric Acid
Physiology / Gastrointestinal / Stomach
Last Updated: 24th February 2020
The acid environment in the stomach provides a defence mechanism against ingested microorganisms, aids protein digestion by activation of pepsinogen and denaturation of dietary protein, and stimulates flow of bile and pancreatic juice.
Mechanism of Secretion
In the parietal cells, CO2 and H2O are converted to H+ and HCO3- catalysed by carbonic anhydrase.
H+ is secreted into the lumen of the stomach in exchange for K+ by the H+/K+ ATPase pump (proton pump) on the apical membrane of the parietal cell. Cl- is secreted along with H+; thus the secretion product of the parietal cells is HCl.
The HCO3- produced in the cells is absorbed into the bloodstream in exchange for Cl- (Cl-/HCO3- exchange) across the basolateral membrane. Eventually, this HCO3- will be secreted in pancreatic secretions to neutralise H+ in the small intestine.
Factors Stimulating Secretion
The secretion of HCl is stimulated by:
- Vagal stimulation
- Directly through the neurotransmitter acetylcholine acting at muscarinic (M3) receptors on the parietal cell
- Indirectly through the neurotransmitter gastrin-releasing peptide (GRP) stimulating release of gastrin from G cells
- Histamine acting at H2 receptors on the parietal cell (released from enterochromaffin-like cells in response to vagal activity and gastrin)
- Gastrin released from antral G cells acting at the CCKB receptor on parietal cells
Factors Inhibiting Secretion
The secretion of HCl is inhibited by:
- Low pH (< 3) in the stomach
- Prostaglandins
- Somatostatin (both directly through inhibition at parietal cells and indirectly through inhibition of gastrin and histamine release)
- Enterogastrones
- Secretin
- Gastric inhibitory polypeptide (GIP)
- Cholecystokinin
- Vagotomy
- Drugs
- H2-receptor blockers e.g. cimetidine
- Proton pump inhibitors e.g. omeprazole
- Muscarinic receptor antagonists e.g. atropine
Gastric Acid Secretion. (Image by unknown [Public Domain], via Wikimedia Commons)
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
