physiology of the small intestine and exocrine pancreas Flashcards
what are the 3 parts of the small intestine
duodenum
jejunum
ileum
what does the small intestine receive
chyme from the stomach
pancreatic juice from the pancreas
bile from the liver and gall bladder
what does the small intestine secrete
intestinal juice ie succus enteric
what does motility in the small intestine cause
> mixing of chyme with digestive juices (segmentation)
slow propulsion of the chyme aborally (peristalsis)
removal of undigested residues to the large intestine via the ileocaecal valve (the migrating motor complex)
how is the surface area increased in the small intestine
by: circular folds (Kerkring), villi and microvilli
what is segmentation
mixing of chyme with digestive juices in digestive state
how does segmentation work
chopping moves chyme back and forth - this is particularly vigorous after a meal (almost none between meals)
>due to the alternation contraction and relaxation of segments of circular muscle
what initiates segmentation
small intestine pacemaker cells causing the BER (basic electrical rhythm?) which is continuous
>at threshold this activates segmentation which in the duodenum is primarily due to distension by entering chyme
what has the most segmentation contractions - duodenum or ileum ?
duodenum = 12 ileum = 9
per minute
how can segmentation be enhanced/reduced
via parasympathetic and sympathetic activity
what triggers segementation in an empty ileum
triggered by gastrin from the stomach
how does peristalsis in the inter digestive state occur as
> a few localised contractions
>the migrating motor complex (MMC)
features of the MMC
> it occurs between meals every 90-120mins
strong peristaltic contraction slowly passing length of the intestine (stomach to the ileocaecal valve)
clears small intestine of debris, mucus and sloughed epithelial cells ‘housekeeper function’
inhibited by feeding and vagal activity
triggered by motilin, suppressed by gastrin and CCK
what does the small intestine secrete into the blood
succus entericus and various peptide hormones (from endocrine cells in the mucosa) >gastrin >secretin >cholecystokinin CCK >gastric inhibitory peptide GIP >GLP-1 glucagon like peptide-1 >motilin >ghrelin
gastrin
from G cells of gastric antrum and duodenum
>stimulates hydrogen ion secretion by gastric parietal cells
>stimulates growth of gastric mucosa
secretin
> released in response to hydrogen ions and fatty acids in lumen
promotes secretion of pancreatic and biliary bicarbonate
CCK
> released in response to monoglycerides, free fatty acids, amino acids, and small peptides in the lumen
inhibits gastric emptying
causes secretion of pancreatic enzymes required for digestion
stimulates relaxation of sphincter of Oddi and contraction of gall bladder to eject bile into duodenum
potentiates the action of secretin
GIP
> released in response to glucose, amino acids and fatty acids
stimulates release of insulin from pancreatic B-cells (Incretin action)
inhibits gastric emptying
GLP-1
> stimulates insulin secretion
inhibits glucagon secretion from pancreatic alpha-cells
decreased gastric emptying and appetite
motilin
> is secreted during the fasting state
>initiates the migrating motor complex
ghrelin
> stimulates appetite
what are the control mechanisms of the secretion of intestinal juice
> distension/irritation, gastrin, CCK, secretin, parasympathetic nerve activity enhance secretion
sympathetic nerve activity decreases secretion
secretion of intestinal juice lacks digestive enzymes - what does it contain
mucus
aqueous salt
what does the endocrine pancreas secrete
insulin and glucagon - secreted to blood
what does the exocrine pancreas secrete
digestive enzymes (acinar cells), aqueous sodium bicarbonate solution (duct cells) and these are secreted to the duodenum collectively as pancreatic juice
what are duct cells
> they secrete alkaline fluid into the duodenum per day
they neutralise acidic chyme entering the duodenum
-provides optimum pH for pancreatic enzyme function
-protects the mucosa from erosion by acid
duct cells - what is going on at the apical membrane facing the lumen
> bicarbonate secretion occurs via a calcium/bicarbonate exchanger in the lumen is needed for exchange
CFTR is a chloride channel that provides an important fraction of the luminal chlorine needed for chlorine/bicarbonate exchange - activated by secretin
duct cells - what is going on at the basolateral membrane facing the interstitium
> the sodium/chloride contransporter provides some of the bicarbonate required for its secretion across the apical membrane
sodium/potassium ATPase maintains the electrochemical gradients of sodium and potassium ions
sodium/hydrogen exchanger contributes to the export of hydrogen ions liberated by dissociation of carbonic acid
proton pump contributes to export of hydrogen ions liberated by dissociation of carbonic acid
what can pancreatic enzymes do
can completely digest food in the absence of all other enzymes
what are the pancreatic enzymes and where are they stored
- trypsinogen chymotrypsinogen procarboxypeptidase A and B
pancreatic amylase
pancreatic lipase - stored in zymogen granules of acinar cells and are released as a result of elevated calcium ions
where is enterokinase stored
mucosal cells of duodenum
what does enterokinase cleave
> trysinogen to trypsin
chymotrypsinogen to chymotrypsin
procarboxypeptidase A and B to carboxypeptidase A and B
what are the three phases of control of pancreatic secretion
cephalic
gastric
intestinal
