Stomach Flashcards
What are the 3 key functions of the stomach?
- Digestion of macronutrients - chemically and mechanically
- Storage reservoir
- Immunological protection - acid destroys ingested pathogens
Stomach mucosa is lined with columnar epithelia. They also invaginate into what?
Gastric pits.
Gastric pits contain specialist exocrine and endocrine cells.
What do the fundus and body of the stomach secrete?
Gastric acid (HCL, Mucous, Pepsinogen).
Cardia part of stomach and pyloric canal only secrete mucus.
What does the pyloric Antrum secrete?
Gastrin
How does the stomach differ to other parts of the digestive tract?
It has an extra oblique layer of smooth muscle inside circular layer - aids mechanical digestion.
Explain rugae and their significance.
Empty state - mucosa and submucosa are in folds called rugae.
After eating, rugae are stretched and become flat.
Rugae allows stomach to take its reservoir function.
What border does the Z line represent?
Epithelial Border between oesophagus and stomach.
Below Z line = simple columnar
Above Z line = stratified squamous
Contrast the oesophageal epithelium and the stomach lining.
Oesophageal epithelium - light pink. Function as wear and tear lining.
Stomach lining - bright red. resistant to low pH
Millions of gastric pits line the stomach, which are pores in stomach mucosa containing gastric glands. What do gastric glands house?
Functional secretory cells of stomach.
Secrete gastric juice, mucous, paracrine signalling molecules and hormones.
Describe the cells present in gastric glands.
- Mucous cells
- Parietal cells
- Chief cells
- G cells
- Enterochromaffin-like cells (ECL), D-cells, Gastric stem cells
Describe mucous cells.
Mucous cells - many present. Secrete bicarbonate rich mucous, protecting stomach lining. Keeps pH next to lining near to 7 (combats acidic environment). Also protects stomach lining from active lipase and proteases (which may interfere with lipid bilayer and membranous transporters).
Describe parietal cells.
Parietal cells - acid secreting cells. Quiescent until activated. Activation causes tubovesicles fuse with apical surface to make canalicular surface with high SA for secretion. Many mitochondria for membrane transport. Secrete intrinsic factor (GP for B12 absorption - lack of can cause pernicious anaemia).
What are the function of HCL?
- kill ingested pathogens
- Activate protease zymogens
- Alter protein structure to help digestion.
Describe chief cells
Produce protease zymogen (pepsinogen) and lipase (gastric lipase). Pepsinogen activated to pepsin in HCl presence (gastric lumen). Pepsin then breaks dietary proteins into peptide chains. Gastric lipase digests fats by removing fatty acid from triglyceride molecule.
Why do chief cells secrete pepsinogen as an inactive zymogen?
To prevent autodigestion of themselves.
Describe G cells
Enteroendocrine cells - found in bottom of gastric pit. Release gastrin hormone into bloodstream in response to vagus nerve stimulation/peptides in stomach/stomach distension.
Describe gastrin action.
Travels through blood to receptor cells in stomach. Here it stimulates gastric secretion and motility. Smooth muscle stimulation by gastrin = stronger contractions and pyloric sphincter opens to move food into duodenum.
How does gastrin increase secretion of pancreatic juice and bile?
Binds to receptors on cells in the pancreas and gall bladder.
Describe enterochromaffin like cells (ECLs)
Type of neuroendocrine cell found deep in gastric glands (vicinity of parietal cells). Secrete histamine - stimulates acid secretion from parietal cells.
Describe D-cells.
Enteroendocrine cells secreting somatostatin (inhibitory effect on GI function). Somatostatin also inhibits ECL histamine production and parietal cell activity. Both inhibit HCL secretion.
Describe Gastric stem cells.
Pluripotent cells - capable of differentiating into all different cells of the stomach (various GFs).
Describe how HCl forms.
- CO2 diffuses into parietal cell.
- Carbonic anhydrase = carbonic acid. Dissociates to bicarbonate and proton.
- Bicarbonate exchanges for Cl- in interstitial space.
- Cl- moves into cell via Cl- channels.
- H+ must be pumped into lumen. Achieved via Na/K exchanger (basolateral membrane) and Cl- channel (apical membrane)
- K+ enters cell (Na+ leaves). K+ entry means H+ efflux.
- K+ then reenters lumen via K+ channels.
- Secreted H+ combines with Cl-. HCl.
How does lipase work on triglycerides?
Converts them into a fatty acid and diglyceride.
What are the 3 distinct phases of stomach activity?
- Cephalic phase. Vagus nerve (ACh) stimulates secretion from mucous cells, parietal cells, chief cells, G cells. All via submucosal plexus.
- Gastric phase. Distension of stomach and chemoreception of nutrients and reduced pH. Vagus nerve stimulates mucous, parietal, chief and G cells via submucosal plexus. Increase motility (mixing waves) via myenteric plexus. 3-4 hrs.
- Intestinal phase. Duodenal stretch, chemodetection of reduced pH and duodenal distension. I-cells secrete cholecystokinin, S-cells secrete secretin into blood. Decreases parietal secretion, inhibits gastric motility and emptying. Stretch receptors input into enteric NS, reduces stomach activation. Gastric emptying slows to allow downstream organs to deal with current contents.
