5. Function of the stomach Flashcards
name 2 features of the stomach enabling food storage function
- RUGAE - muscoa/submucosa thrown into folds
- RECEPTIVE RELAXATION of orad stomach (enabled by rugae) - vagally mediated relaxation of upper stomach muscle to allow food entry without too great an increase in intra-gastric pressure
name 2 features of the stomach enabling food disruption for digestion
- 3 layers of SMOOTH MUSCLE (outer longitudinal, middle circular & inner oblique)
- sustained contractions in orad stomach to create basal tone
- strong peristalsis in caudad stomach to mix content (stronger waves as approach closed pylorus) - Stomach FUNNEL SHAPE (larger to smaller) - contents accelerates as moves down, enabling separation of lumps (left behind) and liquid chyme (ejected into duodenum 3x/min)
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describe the 3 layers of the gastric mucosa
- simple columnar epithelium
- lamina propria
- muscularis mucosa
what do gastric epithelial cells secrete to aid digestion - name 3 functions of this product
Epithelial PARIETAL CELLS secrete GASTRIC ACID (HCL):
- unravels proteins - creates larger surface area for enzymes to act on
- activates proteases (pepsinogen to pepsin) for protein breakdown (localises proteases to stomach which has protective mucus layer)
- disnfects stomach content
which products are secreted by the following gastric pit cells:
- mucous cells (surface and neck)
- parietal cells
- enterochromaffin-like cell
- chief cell
- D cell
- mucous cells: mucus
- parietal cells: HCl, HCO3- and intrinsic factor
- enterochromaffin-like cells: histamine
- chief cells: pepsinogen
- D cells: somatostatin
describe the production of HCL by parietal cells
Carbonic anhydrase in parietal cells converts CO2 + H2O into HCO3- + H+:
- H+ actively transported across apical membrane into stomach via HK ATPase
- HCO3- passively transported into blood by anion transporter on basolateral membrane (later transported into duodenum lumen to neutralise previously generated gastric acid
name 3 stimuli which directly induce HCL secretion by parietal cells - how are these coordinated?
3 stimuli have synergisitic effects:
- ACh - released by parasympathetic fibres in vagus n. efferents (synapse directly onto parietal cells)
- histamine - released be enterochromaffin cells lying directly adjacent to parietal cells (in oxynytic gland)
- gastrin - released by G cells (in pyloric glands)
Vagus n. helps coordinate 3 stimuli: n. efferents synapse on G cells to induce gastrin release and enterochromaffin cells to induce histamine release.
name 3 factors that stimulate gastrin secretion from G cells into blood
- peptides/amino acids in stomach lumen
- stomach distension
- vagal stimulation (ACh and gastrin-releasing peptide (GRP))
how is HCL secretion inhibited
- decreased stomach distension… decreased vagal activity
- G cell inhibition: food (acts as buffer) leaves stomach… decreases pH… activates D cells to release somatostatin… inhibits G cells (and ECL cells)
describe the 3 phases of digestion/HCL secretion and how each is stimulated
- cephalic phase (30%)
Initiated by: sensory experience of seeing and eating food… parasympathetic stimulation (vagus n.) of parietal cells and G cells.
- gastric phase (60%)
Initiated by:
- stomach distension… vagus n. stimulation… parietal and G cell stimulation
- presence of amino acids and peptides… G cell stimulation
- food in stomach acting as pH buffer… removes somatostatin-mediated inhibition of gastrin production
3. intestinal phase (10%)
Initiated by: detection of partially digested proteins/chyme in duodenum… gastrin secretion
Soon overtaken by G cell inhibition:
- lipid presence activates enterogastric reflex… reduces vagal stimulation
- chyme stimulates CCK and secretin - helps suppress secretion
name 3 mechanisms for the prevention of gastric auto-digestion
- mucus production and HCO3- release by surface and neck mucous cells (gastric glands): forms thick alkaline viscous layer that adheres to epithelium - epithelial surface kept at high pH
- high turnover of epithelial cells: helps keep epithelia intact
- prostaglandins: maintain mucosal blood flow, supplying epithelium with nutrients
why do alcohol and NSAIDs increase stomach susceptibility to acid
Alcohol: dissolves mucus layer
NSAIDs: inhibit prostaglandins
how does gastric emptying occur
When pressures created by gastric peristaltic waves exceed closing pressure of pylorus muscle, i.e.
- increased peristaltic pressure
- relaxing of pyloric sphincter
under which conditions would gastric emptying be delayed and how is this regulated
Delayed if excessive acid or undigested fats in duodenum. Regulated by:
- nerve reflex pathways that sense duodenal pH: decreased pH… decreased strength of gastric peristaltic contractions and increase pylorus tone
- duodenal and jejenym epithelial cells release CCK in reponse to lipid presence in small intestine lumen… decrease strength of gasteric peristaltic contractions
