secretions of the intestine, liver, gall bladder & pancreas Flashcards
what is the function of the small intestine?
governs the majority of chemical digestion and absorption of nutrients, electrolytes and water
what are the 3 primary secretions of the small intestine?
- intestinal juice
- pancreatic juice
- bile
what key endocrine hormones regulate bile and pancreatic secretions?
- secretin
- cholecystokinin (CCK)
- glucose dependent insulinotrophic peptide/gastric inhibitory peptide (GIP)
what are the 3 main secretory cells in the small intestine and their function?
- villi: absorptive enterocytes and mucus secreting goblet cells
- intestinal glands: enterocytes secreting isotonic fluid, enteroendocrine cells, paneth cells
- Brunner’s glands (duodenum only): secrete mcus and HCO3-
what is contained in the secreted intestinal juice?
electrolytes, water, lysozyme, mucus, alkaline mucus containing fluid
what is contained in the secreted exocrine pancreatic juice?
bicarbonate/digestive enzymes
what are the key endocrine hormone secretions into the vasculature of the small intestine?
- CCK stimulate pancreatic and gallbladder secretion
- secretin stimulates pancreatic and biliary bicarbonate secretion
- GIP may inhibit acid secretion/stimulate insulin release
what does the exocrine pancreas secrete?
pancreatic juice containing bicarbonate rich secretion (pH 8) and digestive enzymes essential for normal digestion and absorption
what is the pancreatic structure?
- the pancreas consists of glandular epithelial clusters
- 99% exocrine acinar clusters secreting pancreatic juice (water, electrolytes, sodium bicarbonate and pro-enzymes)
- 1% endocrine pancreatic islet (islets of Langerhans) of 4 types of secreting glucagon (alpha), insulin (beta), somatostatin (delta) and pancreatic polypeptide (F cell)
how is exocrine pancreatic secretion regulated by acinar enzyme production?
- acetylcholine released via P/S vagus stimulation
- CCK trigger is chyme containing fat and protein products
- produces lower volume enzyme rich pancreatic juice
how is exocrine pancreatic secretion regulated by ductal bicarbonate and water?
- secretin trigger is H+ in highly acidic chyme
- produces copious, HCO3- rich, lower enzyme pancreatic juice
what are the different pancreatic enzymes and their functions?
- proteolytic enzymes: secreted in inactive form, convert proteins to peptides
- amylase: hydrolyses starch, glycogen and other carbohydrates other than cellulose to form di and trisaccharides
- lipase: hydrolyses fat into fatty acids and monoglycerides
- nucleases: digest RNA and DNA to nucleic acids
- trypsin inhibitor: prevents activation of trypsin to prevent pancreatic digestion
how are proteolytic enzymes activated?
- proteolytic enzymes are produced as inactive precursors called zymogens
- small intestinal brush border enterokinase enzyme cleaves hexapeptide to form active trypsin form trypsinogen
- trypsin cleaves and activates other proteolytic enzymes
- process prevents pancreatic auto digestion
how do ducts secreted sodium bicarbonate?
- secretin stimulates high volume HCO3- rich pancreatic juice
- HCO3- secretion out of cell and into duct lumen is via Cl-/HCO3- exchange at the apical cell membrane
- Cl- is recycled out of the cell via the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel under secretin stimulation via cAMP
- Na+ is secreted transcellularly into the duct lumen following HCO3- secretion down electrochemical gradient, water follows by osmosis
what happens with unstimulated channels?
low secretory rate: electrolyte content is similar to that of plasma
what happens with stimulated channels?
higher secretion rate and rise in HCO3- from ductal cells inversely related to reduced concentration of Cl- in pancreatic juice
what happens to patients with a dysfunction in ductal CFTR Cl- channel?
- cystic fibrosis
- lack a functional Cl- CFTR channel in the luminal membrane, which results in defective ductal fluid secretion
- the ducts become blocked with precipitated enzymes and mucus and the pancreas undergoes fibrosis
- blocked ducts impair secretion of needed pancreatic enzymes for digestion of nutrients, resulting in malabsorption
- treatment of this type of malabsorption includes oral pancreatic enzyme supplements taken with each meal
what happens to patients with a dysfunction in the enzyme activation process?
- pancreatitis
- an inflammatory disease where pancreatic enzymes are activated within the pancreas resulting in auto digestion of the tissues
- the most common causes are gallstones and alcohol abuse where obstruction of the pancreatic duct occurs
what are the 2 main roles of bile in digestion?
- required for digestion and absorption of fats from the small intestine
- elimination of waste products
how does bile diets and absorb fats from the small intestine?
bile salts emulsified fats for digestion by pancreatic lipase, solubilise fat digestion products into micelles for absorption across the mucosa
what waste products does bile eliminate?
- bile pigment bilirubin form harm in red blood cell degradation
- cholesterol
- drugs
how is bile synthesised and secreted in the liver?
- bile is constantly synthesised by hepatocytes linen sinusoidal blood vessels in the liver anicus
- hepatocytes are the key functional cell of the liver forming 80% of the liver mass
- bile drains into the blind ended canaliculi and into the bile duct for storage in the gall bladder or direct drainage into the duodenum
how is bilirubin excreted in bile?
- haem from old/faulty RBC is converted to bilirubin and oxidised from biliverdin transported to liver bound to albumin in unconjugated form
- conjugated with glucuronic acid to bilirubin diglucuronide by hepatocytes excreted in bile
- gut bacteria hydrolysis deconugated bilirubin to form urobilinogen
- urobilinogen reduced to stercobilin, secreted in faeces
- enterohepatic reabsorption or urobilinogen most re-secreted in bile
what is jaundice?
the build up of bilirubin
what are the 3 main causes of jaundice?
1) pre-hepatic: excessive RBC breakdown, build up of unconjugated bilirubin due to overload of progressing mechanisms
2) hepatocellular/congenital: altered hepatocyte function
3) post-hepatic: obstruction to normal bile drainage, build up of conjugated bilirubin
what is an example of pre-hepatic jaundice?
haemolytic anaemia
what is an example of hepatocellular/congenital jaundice?
Crigler-Najjar syndrome
what is an example of post-hepatic jaundice?
gallstone obstruction of bile flow
how is bile secreted and regulated?
- CCK released in response to fat content of duodenum: gall bladder contracts and sphincter of hepatopancreatic ampulla relaxation
- secretin released in response to acidic chyme: liver ductal secretion of HCO3- and H20
- minor role for vagal and enteric ACh stimulation: bile flow, gall bladder contraction
how does enterohepatic circulation work with bile salts?
- enterohepatic circulation: bile salts secreted by hepatocytes into bile and continuously recycled through active reabsorption from the ileum and then re-secreted into bile
- 94% bile salts return via portal vein to drive bile synthesis in liver
- many hydrophobic drugs are deactivated by the liver and excreted into bile; enterohepatic recycling frequently occurs; slowing rate of drug elimination
what are the common causes of gallstones?
- excessive water and bile salt reabsorption from bile
- excessive cholesterol in bile causing precipitation
- inflammation of epithelium
what is gall bladder disease?
occurs in several forms, ranging from asymptotic cholelithiasis (gallstones) to biliary colic affecting different areas of the biliary tract
