Lecture 19- Digestion 2 Flashcards
What are the two areas in the stomach where lot of secretion happens?
-oxyntic mucosa= the part of the gastric mucosa in the antrum with a high concentration of glands (75%) -pyloric gland area
What secretion happens in the oxyntic mucosa?
-have chief cells= pepsinogen -parietal cells= HCL, Intrinsic Factor -enterochomaffin-like cells =histamine
What secretion happens in the pyloric gland area?
-different glands here, G cells and D cells in the lower part closer to the pyloric sphincter -G cells=gastrin -D-cells= somatostatin
What is the gastric acid made up of and what is it for?
-HCL= 0.5% and lot of NaCl and KCl -pH 1-2 - key role in digestion of proteins, by activating digestive enzymes, and making ingested proteins unravel so that digestive enzymes break down the long chains of amino acids. -produced by parietal cells
How does the secretion of gastric happen?
-in several steps -H+ and Cl- are secreted separately 1.Chloride and sodium ions are secreted actively from the cytoplasm of the parietal cell into the lumen of the canaliculus. This creates a negative potential of -40 mV to -70 mV across the parietal cell membrane that causes potassium ions and a small number of sodium ions to diffuse from the cytoplasm into the parietal cell canaliculi. 2.The enzyme carbonic anhydrase catalyses the reaction between carbon dioxide and water to form carbonic acid. This acid immediately dissociates into hydrogen and bicarbonate ions. The hydrogen ions leave the cell through H+/K+ ATPase antiporter pumps. 3.At the same time sodium ions are actively reabsorbed. This means that the majority of secreted K+ and Na+ ions return to the cytoplasm. In the canaliculus, secreted hydrogen and chloride ions mix and are secreted into the lumen of the oxyntic gland. stimuli:-Gastrin -Acetylcholine - Histamine
What is pepsin?
-proteolytic enzyme (breaks down protein into peptides) -released in its inactive form by the chief cells=pepsinogen -is activated by HCl -the release of gastrin (hormone) and Actylcholine trigger the release of pepsinogen (as well as HCl)
What triggers the release of HCl?
stimuli:-Gastrin -Acetylcholine - Histamine
What is the gastric mucosal barrier?
-barrier of the stomach that allows it to contain acid -protection of the stomach= mucus coating but wouldn’t work long term, then tight junctions between the cells so the HCl can’t leak into the surrounding tissues -has three components: 1. A compact epithelial cell lining. Cells in the epithelium of the stomach are bound by tight junctions that repel harsh fluids that may injure the stomach lining. 2. A special mucus covering, derived from mucus secreted by surface epithelial cells and Foveolar cells. This insoluble mucus forms a protective gel-like coating over the entire surface of the gastric mucosa. The mucus protects the gastric mucosa from autodigestion by e.g. pepsin and from erosion by acids and other caustic materials that are ingested. 3.Bicarbonate ions, secreted by the surface epithelial cells. The bicarbonate ions act to neutralize harsh acids.
What are the four phases in gastric acid secretion? names only
1.The basal phase 2.Cephalic phase 3.Gastric phase 4.Intestinal phase
What happens in the basal phase of the gastric secretion?
-A small amount of acid is always being secreted into the stomach. The other three following phases increase the secretion rate in order to digest a meal
What happens in the cephalic phase of the gastric secretion?
- Thirty percent of the total gastric acid secretions to be produced is stimulated by anticipation of eating and the smell or taste of food. This signalling occurs from higher centres in the brain through the Vagus Nerve. It activates parietal cells to release acid and ECL cells to release histamine. The Vagus nerve also releases Gastrin Releasing Peptide onto G cells. Finally, it also inhibits somatostatin release from D cells.
What are the ECL cells?
Enterochromaffin-like cells -found in the gastric glands of the gastric mucosa beneath the epithelium, in particular in the vicinity of parietal cells -secrete Histamine -G cells are stimulated by the vagus nerve to release gastrin which stimulates the ECL cells to secrete histamine -histamine and gastrin stimulate the secretion of HCl
What happens in the gastric phase of the gastric secretion?
-stimulated by food entering the stomach -About fifty percent of the total acid for a meal is secreted in this phase. Acid secretion is stimulated by distension of the stomach and by amino acids present in the food.
What happens in the intestinal phase of the gastric secretion?
-Inhibitory : influenced by chyme entering duodenum -The remaining 10% of acid is secreted when chyme enters the small intestine, and is stimulated by small intestine distension and by amino acids. The duodenal cells release entero-oxyntin which acts on parietal cells without affecting gastrin -chyme is starting to be fed into the small intestine= measn fat and acid and high tonicity of material -strong feedback effect = secretes Secretin and CCK(Cholecystokinin) those are called enterogastrones(intestino stomach)- negative effects on the G cells, prietal and chief cells= dampening down of activity in the stomach and ENS and ANS
What is the enterogastric reflex?
eneterogastric reflex= feeding into the ANS= slowing it down
What stops/decreases the gastric secretion in the stomach?
-decrease in Protein & distension in stomach -Very high [H+] (=very low pH)=Accumulates as chyme levels fall in stomach -Enterogastric reflex & enterogastrones