Physiology
Gastrointestinal
Question 26 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
Physiology / Gastrointestinal / Stomach
Last Updated: 21st April 2019
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 |
