Physiology- Intestines, pancreas, liver and gallbladder

Question 1

Describe the function of the three parts of the small intestines

Answer 1

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

Question 2

Which features contribute to increased surface area in small instesines

Answer 2

1. Villi- finger like projections containing blood vessels and lacteal
2. Microvilli- brush border on cells with brush border enzymes for final stages of digestion
3. Crypt villus unit- glands; stem cell division prdouces immature cells which mature at top; secrete fluid

Question 3

Describe the structure of the crypts of Lieberkuhn

Answer 3

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

Question 4

Mucosa contains:

Answer 4

1. absorptive cells
2. goblet cells
3. enteroendocrine cells (secreting secretin, CCK and somatostatin)
4. paneth cell -secretes lysozyme and capable of phagocytosis
5. muscle, lacteal, blood capillary

Question 5

Submucosa contains:

Answer 5

1. Lymphatic vessels
2. Arteriole
3. Venule

Question 6

Describe what happens in celiac sprue

Answer 6

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

Question 7

Which three organs secrete juices into the small intestines?

Answer 7

Gall bladder
Liver
Pancreas

Question 8

1.5L of pancreatic secretions are produced per day. Describe the contents

Answer 8

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

Question 9

Describe the structure of the glands of the pancreas in terms of exocrine and endocrine function

Answer 9

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)

Question 10

In the pancreas there are separate regulatory systems for enzymes and alkaline secretion. Outline each

Answer 10

1. 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)
2. Enzyme: ACh binds to muscarinic receptors on acinar cells, increased blood flow and gastrin release; CCK from duodenal I cells.

Question 11

Outline the mechanism of enzyme secretion by acinar cells

Answer 11

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

Question 12

How is prevention of pancreatic autodigestion achieved?

Answer 12

Produced as inactive precursors = zymogens

Sequestered in membrane- limited vesicles (avoid contact with cytoplasm)

Question 13

Describe how inactive zymogen is activated and how it may be regulated

Answer 13

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

Question 14

What is pancreatitis?

Answer 14

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

Question 15

Outline the mechanism of HCO3- secretion by ductal cells

Answer 15

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-

Question 16

Describe regulation of pancreatic secretion

Answer 16

cephalic and gastric phase account for 10% of regulation

Intestinal phase accounts for 70%

1. Protons from acid stimulate duodenal S cells
2. Monoglycerides, fatty acids and amino acids induce CCK release
3. Lipids and proteins trigger a vago-vagal reflex

Question 17

Describe the function and composition of bile

Answer 17

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

Question 18

Describe the functional structure of the liver

Answer 18

50-100,000lobules (hexagonal units)

Hepatocytes secrete hepatic bile into the blind ended canaliculi- drain into bile ducts, then stored in gall bladder.

Question 19

What happens after bile is used?

Answer 19

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)

Question 20

What is jaundice?

Answer 20

Accumulation of bilirubin in blood

Impaired uptake by hepatocytes; failure to conjugate bilirubin; seen in hepatitis and cirrhosis

Question 21

How is bile release into duodenum following a meal controlled?

Answer 21

MINOR: vagus

MAJOR: CCK (causes relaxation of sphincter of Oddi and gall bladder emptying)

Question 22

State three dysfunctions of the gallbladder

Answer 22

1. Gallstones- blockage of cystic duct (infection of gallbladder )
2. Choledocholithiasis: blockage of CBD
3. Ascending cholangitis: blockage of CBD (associated with infection of bile duct)

Question 23

Where does bile undergo modification?

Answer 23

In bile ducts

As it descends there is an increase in HCO3- as secreted by duct epithelium

Question 24

Describe the difference between bile acid-independant and dependant fraction

Answer 24

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

Question 25

Outline the route of bile

Answer 25

Hepatocytes secrete bile into canaliculi

Canaliculi join together and convey bile towards small terminal ductules at periphery of liber lobules

Bile moves through sequence of progressively larger ducts

Emerges in a hepatic duct

Several hepatic ducts converge to form common hepatic duct

Cystic duct from gall bladder joins CHD to form common bile duct

Question 26

Where is the demarcation between the intrahepatic biliary tree and the extrahepatic biliary tree?

Answer 26

Between sectoral ducts of right liver and right hepatic duct

Between segmental ducts of left liver and left hepatic duct