GI physiology Flashcards
what enzyme digests starch in the mouth?
ptyalin (a type of amylase)
what is responsible for starch digestion in the small intestine?
pancreatic amylase
how are the breakdown products of starch absorbed?
- as monosaccharides via a sodium cotransporter mechanism- on the membrane of enterocytes
- fructose absorbed by facilitated transport
how are proteins digested in the stomach?
pepsin is secreted by chief cells as its zymogen pepsinogen
what is responsible for protein digestion in the small intestine?
- pancreatic enzymes (trypsin and chymotrypsin)
- peptidases lining the small intestinal villi
how are the breakdown products of protein digestion absorbed?
- via a sodium cotransporter mechanism on the luminal membranes of intestinal epithelial cells
- absorbed as di/ tri peptides
how are lipids first broken down in the stomach?
- via lingual lipase
- swallowed w/ saliva along with food
why does emulsification of fats need to occur?
- not water soluble
- and the enzymes that digest them are present in the water
how are triglycerides digested?
via pancreatic lipase
how do micelles form and what is there function?
- bile salts combine with fats
- allows monoglycerides and fatty acids to be transported to the brush border of intestinal epithelial cells
how are the breakdown products of lipids absorbed?
- diffuse into intestinal epithelial cells
- taken up by SER
- released by lymphatic system in the form of chylomicrons
where are bile salts reabsorbed?
terminal ileum
what are the 2 stages in the absorption of glucose in the intestine?
- 2 stage transcellular process
- stage 1- glucose and 2 Na+ in via a Na+/ glucose symport protein
- stage 2- Na+/K+ ATPase transporter (moves Na+ out and K+ in via ATP), GLUT 2 transporter- glucose diffuses out
where do net absorption and secretion occur in small intestine?
absorption- villi
secretion- crypts
by what mechanism is potassium reabsorbed?
passive diffusion
where are Cl- and HCO3- reabsorbed?
ileum and colon
what vitamins are absorbed in the duodenum?
vitamins A and D
what vitamins are absorbed in the jejunum?
vitamins A, D and folic acid
what vitamins are absorbed in the ileum?
folic acid and vitamin B-12
where are parietal cells found and what is their function?
- fundus of the stomach
- secrete HCl and intrinsic factor
where are mucosal cells found and what is their function?
- cardia of stomach
- secrete mucous
where are chief cells found and what is their function?
- found in fundus of stomach
- secrete pepsinogen
what is the function of D cells?
secrete somatostatin
what is the function of enterochromaffin-like cells?
release histamine
what is the function of G cells?
release gastrin
what is the mechanism by which gastric acid is secreted?
- carbonic acid forms
- dissociates into H+ and HCO3-
- chloride shift occurs- chloride into the cell and HCO3- out
- hydrogen ions pumped into lumen via H+/K+ pump
- potassium and chloride ions diffuse into stomach lumen down their concentration gradient
what happens in the cephalic phase?
- via PNS
- sight/ smell/ taste of food and chewing
- ACh released, triggers gastrin and histamine release
- net effect= increased acid production
what happens in the gastric on phase?
- triggered by gastric distention, presences of peptides and amino acids
- gastrin release, acts on parietal and enterocromaffin like cells
- increased acid production
what happens in the gastric off phase?
- low luminal pH inhibits gastrin secretion, indirectly inhibiting histamine release
- stimulates somatostatin release- inhibits parietal cell activity
what happens in the intestinal phase?
- triggered by duodenal distension, low luminal pH, hypertonic luminal contents, presence of amino acids and fatty acids
- triggers release of secretin (inhibits gastrin release, promotes somatostatin release) and cholecystokinin (CCK)- reduces ACh release= secretion of bile by promoting contraction of gall bladder and relaxation of sphincter of Oddi
how are proteases secreted?
- chief cells synthesise pepsinogen
- activated in lumen of stomach due to low luminal pH
- converted from zymogen (pepsinogen) into active form (pepsin)
what is the empty volume of the stomach?
50ml
what is receptive relaxation?
- stomach volume changes without luminal pressure change
- parasympathetic nervous system- vagus (X) causes secretion of NO and serotonin
what is peristalsis?
- wave begins in gastric body
- most powerful in pyloric antrum
- increased by gastrin, gastric distention and PNS
- inhibited by duodenal distention, low duodenal pH, SNS
- pylorus closes when chyme reaches it- antral contests forced back to body for more mixing
what determines the frequency of peristaltic waves?
interstitial cells of Cajal found in muscularis propria
what are the 4 main defences against acid secretion?
- alkaline mucus
- occluding junctions between epithelial cells
- replacement of damaged cells
- feedback loops
