Liver and Pancreas- Physiology Flashcards
what are the 4 broad main functions of the pancreas?
- endocrine function
- secretion of HCO3- and water
- secretion of digestive enzymes
- regulation of exocrine pancreatic secretion
what is chyme? what is its pH?
the stuff that comes out of the stomach, acidic pH
describe the weight of the secretions of the pancreas compared to the weight of the organ itself
secretions are 10x the weight of the organ = massive secretory ability
since the pancreas has both exocrine and endocrine functions, what type of signaling must it utilize amongst its tissues?
paracrine communication between endocrine and exocrine tissues
how do the exocrine secretions of the pancreas leave the pancreas?
through the main pancreatic duct, which fuses with the bile duct at the proximal duodenum, forming the hepatopancreatic ampulla, whose opening is controlled by a valve (hepatpancreatic sphincter)
where does the endocrine portion of the pancreas reside? how much of the pancreas is endocrine-functioning?
in pancreatic islets (islets of Langerhans); accounts for less than 2% of the volume of pancreatic secretions
give 4 endocrine hormones secreted by the pancreas
- insulin
- glucagon
- somatostatin
- pancreatic polypeptide
list and generally describe the 2 components of the exocrine secretions of the pancreas
- aqueous component: rich in bicarbonate (HCO3- to raise the pH of acidic chyme so the lower GI tract is not getting acidic shit)
- enzyme component: accomplishes digestion of proteins, carbohydrates, and fats
what are the 3 parts of the aqueous component of exocrine pancreatic secretions?
- water
- mucus: from goblet cells for protection of tract and lubrication for easy access through small intestine
- bicarbonate
what cells produce which parts of the aqueous component of pancreatic exocrine secretions? in response to what?
ductal cells produce water and bicarbonate; stimulates by secretin
what is secretin? give its main function
secreted by S cells of duodenum in response to high H+ (acidic) and fatty acids in the lumen (from chyme entering the proximal duodenum); acts on ductal cells to produce secretion of alkaline bicarbonate-rich pancreatic secretions with minimal enzyme component
give 3 additional functions of secretin other than acting on ductal cells
- stimulates bile output from liver
- inhibits gastric motility
- inhibits gastrin-mediated effects on parietal cells
what cells produce the enzymatic component of exocrine pancreatic secretions? describe their secretion
acinar cells; primary source of enzymes but only contribute a low volume to exocrine pancreatic secretions (much less than aqueous component)
describe the enzymatic component of exocrine pancreatic secretions (4)
- major proteases: trypsin, chymotrypsin, carboxypeptidase
- major amylases: alpha and pancreatic amylases
- major lipases: pancreatic lipase and phospholipase
- nucleases
break down proteins, sugars, and fats
what stimulates acinar cells to secrete enzymes for pancreatic exocrine secretions?
cholecystokinin!! CCK!!!
what is CCK?
cholecystokinin, secreted by I cells in the duodenum in response to fatty acids, protein, and amino acids that stimulates secretion of the enzymatic component of pancreatic exocrine secretions
what are 2 other functions of CCK other than stimulating the secretion of the enzymatic component of pancreatic exocrine secretions?
- potentiates secretin-driven secretion of bicarbonate (acts on S cells)
- stimulates the gallbladder to release its stored bile
why does the pancreas not digest itself if its making all this digestive shit?
pancreatic enzymes are synthesized, secreted, and stored in inactive forms (zymogens) that will not be bioactive in pancreatic ducts
what are 2 ways in which pancreatic enzymes are activated?
- pH: these enzymes only activate and function within a specific pH range
- duodenal enterokinase/enteropeptidase: produced by enterocytes and converts trypsinogen to trypsin, and trypsin activates many other pancreatic enzymes
other than synthesizing, storing, and secreting the enzymes in an inactive form, how else does the pancreas protect itself from digestive enzymes?
trypsin inhibitor: secreted by pancreatic acinar cells that prevents activation of trypsin inside acinar cells and in ducts, preventing premature activation of digestive enzymes
what regulates the aqueous component of pancreatic exocrine secretion? (4 stimulants and 1 inhibitor)
stimulated by:
1. tactile (mechanoreceptors/stretch, neural control)
2. irritating: presence of feed, neural control
3. secretin: chemical control
4. parasympathetic nervous system (neural control)
inhibited by:
sympathetic nervous system (neural control)
what regulates the enzymatic component of pancreatic exocrine secretions? (3 stimulants and 1 inhibitor)
stimulated by:
1. CCK (chemical control)
2. products of digestion (chemical control)
3. parasympathetic nervous system (neural control)
inhibited by: sympathetic nervous system (neural control)
sum up regulation of pancreatic secretion (5)
- chyme entering the duodenum causes the enteroendocrine cells of the duodenum to release secretin and CCK
- CCK and secretin enter bloodstream
- upon reaching the pancreas, CCK induces secretion of enzyme-rich pancreatic juice and secretin causes secretion of bicarbonate-rich pancreatic juice
- secretin causes the liver to secrete more bile and CCK stimulates the gallbladder to release stored bile
- this is all aided by stimulation of the vagal nerve fibers which also causes release of pancreatic juices and weak contractions of the gallbladder
what would happen if pancreatic protection mechanisms were to fail?
pancreatitis
list 8 of the many functions of the liver (may be deleted if we get into specific LOs)
- lipid metabolism (VLDL, LDL, bile)
- synthesis of clotting factors
- synthesis of plasma proteins
- urea cycle for protein metabolism
- blood detox
- conjugation of bilirubin
- chemical alteration of hormones and drugs
- synthesis of vitamin C (not for primates) and activation of vitamin D
what is meant by first pass metabolism relating to the liver?
all blood leaving the GI tract (distal esophagus to rectum (tubular), pancreas, spleen, and visceral fat) goes to the liver first via the portal vein before entering the rest of systemic circulation; the liver then metabolizes, modifies, and excretes things based on what it sees
what is bile?
an alkaline fluid containing both bile acid and bile salts that aid in absorption of fat and are amphipathic; bile is heavily pigmented due to bilirubin from heme from old RBCs
who secretes bile salts and how?
hepatocytes by active transport
how does bile get to the gallbladder?
bile goes through bile canaliculi to the bile ducts to the gallbladder for lipid digestion
what is bile critical for? (2)
- lipid digestion and absorption
- absorption of fat soluble vitamins
wha is the gallbladder?
a saclike structure that concentrates bile and secretes glycoproteins
how does bile from the liver get to the gallbladder?
via hepatic ducts
what species (7) lack a gallbladder and why?
horses, camels, deer, giraffes, moose, elephants, elk, because they constantly eat so need constant digestion, not stored and released bile at mealtimes
how is bile release regulated?
- acidic, fatty chyme entering the duodenum causes release of CCK and secretin from duodenal wall enteroendocrine cells
- CCK and secretin enter bloodstream
- bile salts and secretin are transported via bloodstream to liver, stimulate liver to produce bile more rapidly
- vagal stimulation causes weak contraction of gallbladder
- CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax, allowing bile to enter duodenum
- bile salts are reabsorbed into blood
describe enterohepatic circulation of bile
bile (95%) and bilirubin (5%) are reabsorbed by the GI tract via the portal vein from the ileum back to the liver
when bile acids are initially made by hepatocytes, are they water soluble or nah?
not water soluble
how do bile acids become bile salts?
bile acids are conjugated (reversibly have an amino acid attached) in the liver and are called bile salts
in the duodenum (pH of 3-5), what happens to conjugated bile acids? what happens to unconjugated bile acids/salts?
unconjugated bile acids become protonated and are not water soluble
conjugated bile acids/salts become deprotonated and ARE water soluble, so can be absorbed
what must bile acids be for efficient function and reabsorption? what does this explain?
water soluble! explains why only the ileum enterohepatic circulation reabsorbs water soluble conjugated bile acids/salts and send it back to the liver via the portal vein even though portal circulation is all over the GI tract (only conjugated, water soluble bile acids/salts can be reabsorbed, and also function and they are only conjugated in the duodenum so can not be reabsorbed before that)
