Physiology- Intestines, pancreas, liver and gallbladder Flashcards
Describe the function of the three parts of the small intestines
Duodenum-receives stomach contents, mixes with bile, pepsin and pancreatic enzymes. Contains brunners glands (secrete HCO3- rich anti-acid) and succus entericus
Jejenum- chemical breakdown and absorption
Ileum- has peyer’s patches which form part of immune system by preventing growth of bugs
Which features contribute to increased surface area in small instesines
- Villi- finger like projections containing blood vessels and lacteal
- Microvilli- brush border on cells with brush border enzymes for final stages of digestion
- Crypt villus unit- glands; stem cell division prdouces immature cells which mature at top; secrete fluid
Describe the structure of the crypts of Lieberkuhn
Maturation zone- cells moving toward tip beginning to express enzymes and absorptive membrane transport proteins
Crypt- rapidly dividing stem cells that force migration of cells up. * source of intestinal fluid secretion
Mucosa contains:
- absorptive cells
- goblet cells
- enteroendocrine cells (secreting secretin, CCK and somatostatin)
- paneth cell -secretes lysozyme and capable of phagocytosis
- muscle, lacteal, blood capillary
Submucosa contains:
- Lymphatic vessels
- Arteriole
- Venule
Describe what happens in celiac sprue
Malabsorption syndrome due to hypersensitivity to gluten and gliadin.
Immune mediated destruction of SI villi leading to malabsorption
Causes diarrhoea, steatorrhoea, bloating and flatulence
Diet changes resolve syndrome but malabsorption continues until crypt cells regenerate absorptive cells
Which three organs secrete juices into the small intestines?
Gall bladder
Liver
Pancreas
1.5L of pancreatic secretions are produced per day. Describe the contents
EXOCRINE Alkaline fluid (approx pH 8-8.3) necessary to neutralise stomach chyme as pancreatic enzymes are neutral
Enzymes
ENDOCRINE
Hormones- insulin and glucagon involved in metabolic regulation
Describe the structure of the glands of the pancreas in terms of exocrine and endocrine function
EXOCRINE
80% acinus which secrete digestive enzymes and duct cells which secretes HCO3-
Secretions delivered to intestinal lumens by pyramidal epithelium
Majority of secretion is proteolytic enzymes, others: lipolytic, amylotic, nucleases
ENDOCRINE
Islets of Langerhans
Secretions delivered to blood stream
Dysfunction: diabetes (1- autoimmune destruction of B cells, 2- insulin resistance)
In the pancreas there are separate regulatory systems for enzymes and alkaline secretion. Outline each
- Alkaline: secretin (by S cells in duodenum) stimulated by low pH as food enters, secretin enters blood-duct cells
(pH is rarely ever low enough to stimulate high levels of secretin release) - Enzyme: ACh binds to muscarinic receptors on acinar cells, increased blood flow and gastrin release; CCK from duodenal I cells.
Outline the mechanism of enzyme secretion by acinar cells
Cell contains nucleus, golgi, ER, zymogen granules
Enzymes are made into inactive form on ribosomes and packaged into zymogen granules (vesicles) at golgi.
Upon secretion of CCK, ACh or secretin, secondary messengers [cAMP] and [Ca2+] cause fusion and exocytosis of zymogen which release into lumen
How is prevention of pancreatic autodigestion achieved?
Produced as inactive precursors = zymogens
Sequestered in membrane- limited vesicles (avoid contact with cytoplasm)
Describe how inactive zymogen is activated and how it may be regulated
Trypsinogen is converted to tryprin by enterokinase which is bound to apical membrane of SI cells
Trypsin converts inactive zymogen to active enzyme
Trypsin conversion can be inhibited by inhibitor produces in pancreas if inappropriately activated
What is pancreatitis?
Pancreatic enzymes activated in pancreas causing inflammation. Causes epigastric pain that radiates from epigastrium to back and is relieved by leaning forward
Main cause in men is alcohol, in women it is gall stones
Other causes: cystic fibrosis, hypercalcaemia or fat
Outline the mechanism of HCO3- secretion by ductal cells
On apical membrane (facing lumen) there is Cl-/HCO3- exchanger which pumps HCO3- into lumen
Na+ channel allows Na+ to follow HCO3- causing H20 to follow
Inward CTFR Cl- channel maintains intracellular Cl- conc.
Ionic concentration of pancreatic juice depends on rate of secretion. Increased rate increases HCO3- and decreases Cl-
Describe regulation of pancreatic secretion
cephalic and gastric phase account for 10% of regulation
Intestinal phase accounts for 70%
- Protons from acid stimulate duodenal S cells
- Monoglycerides, fatty acids and amino acids induce CCK release
- Lipids and proteins trigger a vago-vagal reflex
Describe the function and composition of bile
Its an exocrine secretion of liver but stored in gall bladder. (Between meals bile concentrated 20 fold)
Composition: water, ions, bilirubin and biliverdin (from Hb breakdown in spleen), bile salts
Synthesised from cholesterol
Alkali to neutralise acid
Salts facilitates absorption of fats
Acts as a conduit for excretion of breakdown of blood cell components
Describe the functional structure of the liver
50-100,000lobules (hexagonal units)
Hepatocytes secrete hepatic bile into the blind ended canaliculi- drain into bile ducts, then stored in gall bladder.
What happens after bile is used?
After fat absorption, 25% bile deconjugated (returned back to bile acids) by guy bacteria remains. Some is converted to lithocholic acid by bacteria and lost in faeces
Conjugated bile acids completely reabsorbed by terminal ileum and returned by hepatic portal vein in entero-hepatic circulation. This is IMPORTANT as bile acid pool is not large enough to assimilate the lipid content of a typical meal (recycled approx 2 times per meal)
What is jaundice?
Accumulation of bilirubin in blood
Impaired uptake by hepatocytes; failure to conjugate bilirubin; seen in hepatitis and cirrhosis
How is bile release into duodenum following a meal controlled?
MINOR: vagus
MAJOR: CCK (causes relaxation of sphincter of Oddi and gall bladder emptying)
State three dysfunctions of the gallbladder
- Gallstones- blockage of cystic duct (infection of gallbladder )
- Choledocholithiasis: blockage of CBD
- Ascending cholangitis: blockage of CBD (associated with infection of bile duct)
Where does bile undergo modification?
In bile ducts
As it descends there is an increase in HCO3- as secreted by duct epithelium
Describe the difference between bile acid-independant and dependant fraction
BILE ACID DEPENDENT FRACTION
Hepatic bile made by hepatocytes when sufficient bile acids available. Includes primary bile salts and bile pigments
BILE ACID INDEPENDENT FRACTION
Produced by duct epithelium by secretion of H2O and HCO3-
In intestine, small amount of secondary bile acids formed by bacteria converting small amount of primary acids
