Gastric Secretion Flashcards
What is the main function of the fundus?
- storage
List the functions of the body of the stomach.
- storage
- production of mucus
- production of hydrochloric acid
- production of pepsinogen
- production of intrinsic factors
What are the functions of the antrum?
- mixing and grinding to form chyme
- production of gastrin
Name the three main types of gastric glands and state what they secrete.
- Mucous neck cells - immature mucous cells
- Chief cells - secrete pepsinogens
- Parietal cells - secrete hydrochloric acid and intrinsic factor
State the nine steps of hydrochloric acid secretion from parietal cells.
- CO2 diffuses across the cell
- The CO2 combines with water to form carbonic acid. This is catalysed by carbonic anhydrase
- carbonic acid immediately dissociates to form hydrogen and bicarbonate
- Hydrogen is actively transported out of the cell in an H/K pump
- This decreases the stomach lumen pH
- bicarbonate is transported out the basolateral membrane by an exchanger for chlorine
- This increases the blood pH
- Chlorine exits via a channel at the apical membrane
- Water will pass through the paracellular pathway
Why does hydrochloric acid secretion only take place when eating?
- It uses a lot of energy
Which four hormones regulate hydrogen being released from the lumen and therefore acidity?
- gastrin
- histamine
- acetylcholine
- prostaglandins
How does gastrin trigger secretion of hydrogen?
- gastrin binds to G-protein coupled receptor
- this triggers a rise in calcium
- the calcium reacts with protein kinases
- this increases H/K pump activity
- so more hydrogen is released
How does histamine trigger secretion of hydrogen?
- histamine binds to the Gs receptor, which is coupled to an AC receptor
- AC converts ATP to cAMP
- this reacts with protein kinases
- this increases H/K pump activity
- so more hydrogen is released
How does acetylcholine trigger secretion of hydrogen?
- acetylcholine acts on muscarinic cholinergic receptors
- this triggers a rise in calcium
- the calcium then reacts with protein kinases
- this increases H/K pump activity
- so more hydrogen is released
How do prostaglandins trigger a reduction of hydrogen secretion?
- prostaglandin acts on G1 receptors
- this then inhibits the AC receptor
- therefore, cannot produce cAMP to act on protein kinases
- this inhibits the H/K pump
- so less hydrogen is released
Which three mechanisms control gastric acid secretion?
- neurocrine (vagus/local reflexes)
- endocrine (gastrin)
- paracrine (histamine)
How are gastric secretions stimulated in the cephalic phase?
- the sight, smell, taste, or even thinking about food triggers activation of the vagus nerve
- the vagus nerve increases acetylcholine and also G cells which then release gastrin
- these all act on parietal cells
- acetylcholine and gastrin also stimulate the release of histamine, contributing to a positive feedback loop
How are gastric secretions stimulated in the gastric phase?
- distension of the stomach triggers the vagal and enteric reflexes which release acetylcholine
- peptides in the lumen trigger G cells to release gastrin
- the increase in acetylcholine and gastrin stimulate the release of histamine
How are gastric secretions inhibited in the cephalic phase?
- vagal activity decreases as soon as you stop eating
