Secretions of the intestine, liver, gallbladder and pancreas Flashcards
Duodenum
Receives stomach contents, pancreatic juice and bile
Neutralises stomach acids
Emulsifies fats
Pepsin inactivated by pH increase
Pancreatic enzymes
Jejunum
Most nutrients absorption occurs here
Ileum
Has peyer’s patch- aggregated lymphoid nodules
Function of small intestine
Responsible for the completion of the chemical digestion of ingested food and subsequent nutrient absorption
Produces succus antericus and receives secretions from the liver/ gall bladder and exocrine pancreas
Small intestine surface area
Circular folds
Villi: fingerlike projections 1mm tall, contain blood vessels and lymphatics
Microvilli: cover surface, brush border on cells, brush border enzymes for final stages of digestion
Crypt- villus unit
At villus tip, enterocytes fully differentiated for absorption of nutrients, electrolytes and fluids
Maturation zone intermediate zone where cells move towards tip, beginning to express enzymes
Crypt contains rapidly dividing stem cells that force migration of cells up the side of villus
Celiac sprue
Malabsortion syndrome caused by hypersensitivity to wheat, gluten and gliaden
Immune mediated destruction of the small intestinal villi
Function of pancreas in digestion
Secretes about 1.5l of juices
Secretes alkaline fluid that neutralises chyme
Secretes enzymes that break down macromolecules in food
Functional anatomy of panreas
Exocrine gland made up of acinus and cut cells
- secretions delivered to duodenum via pancreatic duct
- 1-2L pancreatic juice secreted per day
Endocrine glands
- 4 types of islet cells release hormones
Diabetes
Pancreas does not make enough insulin or body cell’s do not use insulin effectively
Composition of pancreatic juices
Proteolytic enzymes
Lipolytic enzymes
Amylolytic enzymes
Nucleases
Others
Regulation of pancreatic secretions
Enzyme secretions:
- ACh- binds muscarinic receptor on acinar cells, increases blood flow and gastrin
- CCK released from duodenal I cells when food enters duodenum
Alkaline secretions:
- secretin- by S cells in duodenum stimulated by low pH
- pH rarely low enough to stimulate high levels of secretin
Mechanisms of enzyme secretions by acinar cells
Pancreatic enzymes synthesised as inactive proenzymes
On stimulation by agonists, release of content into lumen via intracellular 2nd messenger
Fusion and exocytosis of zymogen granules
Move to apical membrane, fuse with plasma and discharge content into luminal space
Prevention of pancreatic autodigestion
Most enzymes produced as inactive zymogens
Sequestered in membrane limited vesicles, avoiding contact with acinar cell cytoplasm
Activation depends on trypsin, pancreas produces a trypsin inhibitor
Pancreatitis
Pancreatic enzymes activated within pancreas so autodigestion of tissue
Caused by gallstones mainly in women and alcohol mainly in men
Epigastric pain radiating from epigastrium to back relieved by leaning forward
Mechanism of isotonic NaCl primary secretion by acinar cells
- NaK pump creates inwardly directed Na+ gradient across basolateral membrane
- Na/K/Cl cotransporter produces net Cl- uptake, driven by Na+ gradient, generated by NaK pump
- Rise in intracellular K+ results from activity if pump and cotransporter is shunted by basolateral K+ channels for exit pathway for K+
- Intracellular accumulation of Cl- makes electrochemical gradient driving Cl- secretion into acinar lumen through apical membrane Cl- channels
- Movement of Cl- into lumen makes transepithelial voltage more lumen negative, driving Na+ into lumen via tight junction
Secretions of pancreatic duct cells
Secrete 1-2L alkaline fluid into duodenum per day
Neutralises chyme entering duodenum
- provides optimum pH for pancreatic enzyme function
- protects mucosa fro erosion by acid
Mechanism of HCO3- secretion by ductal cells
- HCO3- from cell cytoplasm into lumen via Cl-/HCO3- exchange in luminal cell membrane
- Cl- recycled from lumen into cell via CFTR Cl- channel
- Na+ secreted into duct lumen following HCO3- secretion, water follows by osmosis to make fluid secretion
Ionic composition of pancreatic juice depends on rate of secretion
Greater rate of secretion, higher the HCO3- and lower the Cl-
In unstimulated state- flow is low and electrolyte composition similar to plasma
Stimulated state- flow rate increases and rise in HCO3- matched by decrease in Cl-
Dysfunction in ductal Cl- channel
Cystic fibrosis
Lack functional Cl- in luminal membrane
Duct becomes blocked with precipitated enzymes and mucus and pancreas undergoes fibrosis
Blocked ducts impair secretions of pancreatic enzymes resulting in malabsorption
Bile functions
Provides alkali to neutralise acid
Provides bile salts to facilitate absorption of fats
Acts as vehicle for excretion of breakdown components
Composition of bile
Water
Ions
Bilirubin and biliverdin
Bile salts
- synthesised from cholesterol
- required for emulsification and absorption of lipids, cholesterol and phospholipids
Organisation of liver
50-100,000 lobules (1-2mm functional unit)
hepatocytes secrete hepatic bile into blinded ended canliculi- drain into bile ducts then stored in gall bladder
Gallbladder and bile
Sac on underside of liver
500-1000mL bile secreted daily from liver
Stores and concentrates bile
Yellow-green fluid containing minerals, bile acids, cholesterol, bile pigments and phospholipids
Biliary tree
Hepatocytes secrete bile into canaliculi
Canaliculi join and convey hepatic bile towards terminal ductules at periphery of liver lobules
Bile moves through sequence of larger lobules and emerges in hepatic duct
Hepatic ducts from each lobe join outside liver to form common hepatic duct
Network of ducts known as biliary tree
Mechanisms of secretion
Bile made of 2 distinct fractions through different mechanisms
- bile acid dependent (hepatic bile) made by hepatocytes when sufficient bile acids available
- bile acid independent product by duct epithelium by secretion of water and electrolytes
Bile acid dependent fraction- bile salts
Hepatic cells secrete primary bile salts
- from cholesterol metabolism
In intestine, small amount of secondary bile acids formed by bacteria converting small amount of primary acids
Enterohepatic circulation regulates secretion of bile acids
After functioning, 25% bile deconjugated by bacteria during length of ileum
Conjugated bile acids completely reabsorbed by terminal ileum and returned by hepatic portal vein in entero-hepatic circulation
Deconjugated acids returned, fraction converted into litocholic acid by bacteria lost to faeces
Process is 94% efficient
Bile pigments
Acid dependent fraction contains bile pigments
Excretory function
Breakdown of haem in spleen generates insoluble bilirubin
Hepatocytes conjugate bilirubin and excreted via bile
In colon, some conjugated bilirubin converted to urobilirubin for recirculation and excretion via kidneys
Hepatic jaundice
Accumulation of bilirubin in blood
- impaired uptake by hepatocytes
- failure to conjugate bilirubin
- as seen in hepatitis and cirrhosis
Control of bile release into duodenum following meal
Bile release
- minor role of vagus
- major: CCK released from duodenum on presence of fatty and acidic chyme
- CCK stimulates emptying of gall bladder and relaxation of sphincter of Oddi
Dysfunction of gall bladder
Ranges from cholelithiasis ( to biliary colic
