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97

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Physiology

Gastrointestinal

Question 87 of 180

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.

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

VIa Wikimedia Commons

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

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