The Stomach Flashcards
What are the actions of the stomach
Food storage
Contracts rhythmically to mix and disrupt food
Secretes mucosa to protect its wall
Produces hypertonic chyme
Some absorption
Infection control
Secretes some intrinsic factor
Describe receptive relaxation
Receptive relaxation allows the stomach to store food
Vagally mediated relaxation of the stomach allows food to enter the stomach without raising the intra-gastric pressure too much
This prevents reflux during swallowing
What functions does the acidic condition of the stomach have in protein digestion
Acidic conditions are required for pepsinogen to be converted to pepsin - ensures pepsin is only activated once within a safe environment
Acidic conditions denatures proteins -> increases SA of the proteins to increase digestion
Name the parts of the stomach
Fundus - top of stomach
Body - thin walled
Antrum - thick walled
Pylorus - smooth muscle sphincter
What tissues are found in the stomach
Mucosa - secretes acid, digestive enzymes and gastrin
Muscularis mucosae
Submucosa
Muscularis externa - oblique, circular and longitudinal
How does the funnel shape of the stomach aid in digestion
Funnel shape causes contents to accelerate as they move down towards the sphincter
This causes the contents to separate based on size - large molecules are left behind
This means the duodenum is not overwhelmed by large molecules
What cells are found in the gastric glands
Mucous cells - surface and neck cells
Parietal cells - produce intrinsic factor. Secrete H+ into lumen and HCO3- into capillaries
Chief cells - secrete pepsinogens
Enterochromaffin cells - secrete histamine
G cells - secrete gastrin
D cells - secrete somatostatin
Name and describe two hormones in the gastrin family
Gastrin - produced by G cells. Increases gastric acid secretion. Similar in structure to CCK -> tends to bind to CCK receptors
Cholecystokinin - produced by I cells. Production stimulated by fat and protein. Increases pancreatic and gallbladder secretions. Causes gallbladder to contract
Name and describe two hormones in the secretin family
Secretin - produced by S cells. Production stimulated by H+ and fatty acids. Increases HCO3- secretion and decreases gastric acid secretion
Gastric inhibiting polypeptide - produced by cells in duodenum and jejunum. Increases insulin secretion and decreases gastric acid secretion
Describe how HCl is produced in parietal cells
CO2 and water react to form carbonic acid - catalysed by carbonic anhydrase
Carbonic acid dissociates and the H+ is pumped out fo the cell against a gradient using a H-K-ATPase pump
Bicarbonate is then pumped out into the venous drainage via an anion antiport protein that transports Cl- into the cell
Cl- is then transported into the stomach via Cl- channel protein
How is HCl production controlled
Parietal cells are stimulated by gastrin, histamine and ACh
Histamine allows fine tuning of acid release
HCl production is inhibited by somatostatin which inhibits the production of histamine and gastrin
How is gastrin secretion controlled
G cells secrete gastrin
G cells are stimulated by peptides/amino acids in the stomach, distension of the stomach and vagal stimulation
Somatostatin inhibits gastrin production
What stimulates the production of somatostatin and where is it produced from
Low pH
D cells produce somatostatin
What are the phases of digestion - describe them
Cephalic - occurs before food reaches stomach. PNS stimuli causes vagus nerve to stimulate parietal and G cells
Gastric - occurs once food reaches stomach. 60% of HCl production occurs during this phase. Food acts as a buffer -> removes inhibition on gastrin production
Intestinal - has 10% of HCl production but mostly inhibits HCl production.
What are the stomach defences
Mucus/HCO3- - forms thick, alkaline viscous layer on surface of epithelium to protect against acid and enzymes
High turnover of epithelial cells - keeps epithelium intact
Prostaglandins - maintains mucosal blood flow
