Gastric secretions Flashcards
Which cells do the stomach release gastric secretions from
Gastric mucosa
Where are the oxyntic mucosa found
In the fundus and body
Where are the pyloric gland area found
In the antrum
where are mucosal gland cells found
IN gastric pits
What cells in the stomach produce HCL
Parietal cells
What else do parietal cells produce
Intrinsic factors which are involved in vitamin B12 production
What cells in the stomach are responsible for producing pepsinogen
Chief cells
What cells are responsible for secreting mucous
Surface epithelial and mucous neck cells
How is HCL produced by parietal cells
1) carbonic anhydrase catalyses Co2 with water to produce carbonic acid
2) This then dissociates into bicarbonate and H+
3) H+ leave parietal cell via proton pump into gastric lumen in exchange for K+ coming into parietal cell
4) Bicarbonaye ions transporter moves it out of cell into blood and chloride moves into parietal cell
5) Chloride then moves out of cell into lumen via a cotransporter while the K+ moves in from lumen
How does HCl aid digestion
activating the enzyme pepsin and providing an acid medium for optimal pepsin activity
Aids the breakdown of connective tissue and muscle fibres
What produces pepsinogen
Chief cells
How is pepsinogen activated
The acidic environment caused by HCL turns pepsinogen into pepsin which breaks down protein and produces smaller and smaller peptide fragments
Pepsin can also break down pepsinogen to form more pepsin
When is the cephalic phase stimulated by and what is it
Gastric secretion stimulated prior to the arrival of food in the stomach
-stimulated by the thought or expectation of food and the sight, taste, chewing or swallowing of food and the sight, taste, chewing or swallowing of food
What nerve is the action of the cephalic phase dependant on
Vagus nerve
How does enterochromatin cells contribute to the release of HCl
Enterochromatin cells receive ACh on its muscarinic receptors which releases histamine
-Histamine acts on H2 receptors on parietal cells which then release HCL
How do Antral G- cells contribute to the release of HCl
Innervated by parasympathetic nn system. When activated it releases gastrin releasing polypeptide which causes release of gastrin from Antra G cells
-It passes into capillary cells and binds to CCK-2 receptors on enterochromatin like cells which releases more histamine which binds to H2 on parietal cell which releases more HCL
How do D cells affect HCl secretion
Under normal conditions they release somatostatin (SS) which suppresses release f=of gastrin. Activated by parasympathetic nervous system which stops the release of SS from D cells so more gastrin released from antra cell
What is the gastric phase and why
When food reaches stomach, gastric secretion is further increased
- Local vago-vagal reflexes in response to mechanical stimulation, due to presence of food in the antrum (doesn’t involve the CNS)
- Stimulation of gastrin release from G-cells by peptide fragments of partially digested proteins
How does gastric circulation increase when eating a meal
-Local metabolite production (prostaglandins maintain capillary network) may be release of vasoactive intestinal polypeptide (VIP) from local nerves causing vasodilatation
As a meal is gradually passed to small intestine, how are gastric secretions reduced
Removal of peptide fragments (no longer present to stimulate gastrin release)
Removal of food leads to a decrease in pH. At pH less than 2, gastrin release from G cells is inhibited directly and via somatostatin release
What role does the duodenum play in inhibiting gastric secretion
Once food passes into small intestine, they stretch.They sample the contents of the duodenum and sense the acidity. They can sense if fat is present. If there is fat or acid present, they release enterogastrone which inhibits the release of gastrin
What cephalic factors influence gastric secretion
Fear
Depression
