Physiology of the Small Intestine Flashcards
what are 5 things that the Exocrine Pancreas and Small Intestine secrete
hormones alkali digestive enzymes water mucus
where is mucus secreted from the small intestine
Brunner’s glands in the duodenal mucosa secrete a viscous, mucus-rich alkaline fluid.
How is water secreted by the small intestine
Intestinal epithelium secretes Na+, Cl- and HCO3- into the lumen. Water follows by osmosis.
N.B. overall there is a large net absorption of water from the small intestine.
what hormones are secreted by the small intestine
Secretin, cholecystokinin (CCK), motilin, vasoactive intestinal peptide (VIP) and gastric inhibitory peptide (GIP).
what is motilin
Stimulates migrating motor complexes (MMCs) via both the enteric and autonomic nervous systems.
what are migrating motor complexes
a distinct pattern of electromechanical activity in gastrointestinal smooth muscle inbetween meals. It is thought to serve a “housekeeping” role and sweep residual undigested material through the digestive tube.
what is VIP and what does it do
Vasoactive intestinal peptide
increases blood flow to the GI tract
what is GIP and what does it do
Gastric inhibitory peptide
glucose-dependent insulinotropic peptide
Inhibits gastric secretion.
Stimulates insulin secretion.
what is CCK
Cholecystokinin
inhibiting gastric emptying.
what does CCK do
works on the exocrine pancreas and stimulates it to release bicarb ions
Also stimulates the liver to secrete bile
what does secretin do
on exocrine panc stimulating release of pancreatic enzymes. Also acts on gallbladder .’ contracts and release its contents into small intestine
where are exocrine pancreatic enzymes released from
acinar cells
what do acinar cells secrete
exocrine pancreas enzymes
what do duct cells of the exocrine pancreas secrete
alkali
what is the alkali of the exocrine pancreas
An isotonic solution rich in HCO3-.
what is the function of exocrine pancreatic alkali
Neutralizes duodenal contents.
that digestive enzymes does the exocrine pancreas secrete
Trypsin (endopeptidase) Chymotrypsin (endopeptidase) Carboxypeptidase (ectopeptidase) Pancreatic amylase Lipases Other enzymes (phospholipase, cholesterol esterase, ribonuclease etc)
what do lipases do
break down fats
what does pancreatic amylase do
breaks down starches
what does trypsinogen released from the pancreas do
trypsinogen -> trypsin which cleaves the zymogens released from the pancreas to produce active enzymes that can work at the intestinal lumen
Why are peptidases secreted in their inactive form?
So that they don’t break down the cells that are releasing them
when is pancreatic HCO3- secreted
majority of pancreatic secretion arises from intestinal phase stimuli (when chyme reaches the duodenum)
how does body know to increase pancreatic HCO3- secretion
majority of pancreatic secretion arises from intestinal phase stimuli (when chyme reaches the duodenum)
secretin and cholecystokinin (CCK) are released by endocrine cells that are located in the duodenal epithelium.
how does secretin trigger bicarb release
secretin release is triggered by H+ ions (low pH). Secretin then travels via the circulation to stimulate bicarbonate secretion by duct cells.
what is the mechanism of pancreatic HCO3- secretion
In CSF have this complex reaction of 3CO + 3H2O catalysed by carbonic anhydrase to give rise to bicarb ions and H+ ions.
HCO3- are secreted into the duct lumen.
H+ are pumped out of duct cells across basolateral membrane and released into the blood.
how is pancreatic enzyme secretion controlled
stimulated by CCK and acetylcholine (Ach; from parasympathetic, postganglionic neurones of vagus
how is pancreatic alkali secretion controlled
stimulated by secretin and potentiated by CCK and ACh (vagal tone).
how are fats digested in the small intestine
digested by pancreatic lipase to monoglyceride and fatty acids.
how is starch digested
by pancreatic amylase.
Disaccharides are broken down into monosaccharides by brush border enzymes.
how are proteins digested in the small intestine
broken down to peptide fragments by trypsin and chymotrypsin. Peptide fragments are digested to free amino acids by carboxypeptidase (pancreas) and aminopeptidase (luminal membranes of small intestine epithelial cells).
how are fats digested and absorbed
- broken down by emulsification by bile salts and phospholipids
- digested by pancreatic lipase
- products held in micelles, combined with bile salts an phospholipids
- The micelles diffuse into an “unstirred layer” next to the surface of epithelial cells.
what happens to micelles containing fat after they have diffused into the unstirred layer (next to surface of epithelial cells)
fatty acids &monoglycerides diffuse into cell membrane.
- > fatty acids &monoglycerides reassembled into fats.
- > Triglyceride droplets packaged into chylomicrons.
- > Chylomicrons exported across basolateral membrane -> they leave the intestinal villus via its lymph vessel
