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Time Completed: 01:07:02

Final Score 54%

98
82

Questions

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Physiology

Gastrointestinal

Question 128 of 180

Secretion of which of the following substances is inhibited by low pH:

Answer:

The principal physiological function of gastrin is to increase H+ secretion. H+ secretion decreases the pH of the stomach contents. The decreased pH, in turn, inhibits further secretion of gastrin — a classic example of negative feedback. The remaining hormones act to inhibit gastric acid secretion, and thus secretion is not inhibited by low pH.

Gastrin is secreted by antral G-cells and acts on cholecystokinin B (CCKB) receptors.

Stimulating Factors

Gastrin secretion is stimulated by:

  • The presence of small peptides and amino acids in chyme
  • Gastric distension
  • Vagal stimulation directly via acetylcholine and indirectly via gastrin-releasing peptide (GRP)
  • Raised gastric pH

Inhibiting Factors

Gastrin secretion is inhibited by:

  • Low gastric pH (negative feedback mechanism)
  • Somatostatin
  • Secretin
  • Gastric inhibitory polypeptide (GIP)
  • Cholecystokinin

Function

Gastrin acts to:

  • Stimulate acid secretion from parietal cells (both directly and indirectly by stimulating release of histamine from ECL cells)
  • Stimulate pepsinogen secretion from chief cells
  • Increase gastric motility
  • Stimulate growth of gastric mucosa
Hormone Released from: Stimulated by: Inhibited by: Acts to:
Gastrin G-cells in the pyloric antrum The presence of small peptides and amino acids in the stomach, gastric distension, raised gastric pH, vagal stimulation Low gastric pH, somatostatin, secretin, cholecystokinin, gastric inhibitory polypeptide Stimulate acid secretion from gastric parietal cells, stimulate pepsinogen from gastric chief cells, increase gastric motility, stimulate growth of gastric mucosa
Cholecystokinin (CCK) I-cells in the duodenum and jejunum The presence of fatty acids/monoglycerides and small peptides/amino acids in the duodenum Somatostatin Increase production of bile, stimulate release of stored bile, increase pancreatic bicarbonate secretion, stimulate pancreatic enzyme secretion, inhibit gastric emptying, inhibit gastric acid production, stimulate pepsinogen secretion
Secretin S-cells in the duodenum A low pH and the presence of fatty acids in the duodenum Somatostatin Inhibit gastric acid secretion, inhibit gastric emptying, stimulate pepsinogen secretion, stimulate pancreatic bicarbonate secretion, increase pancreatic enzyme secretion, increase bile production
Gastric inhibitory polypeptide (GIP) K-cells in the duodenum and jejunum The presence of fatty acids, amino acids and orally administered glucose in the duodenum Somatostatin Inhibit gastric acid secretion, stimulate insulin release

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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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