Secretions of the intestine, liver, gallbladder and pancreas

Question 1

Duodenum

Answer 1

Receives stomach contents, pancreatic juice and bile

Neutralises stomach acids

Emulsifies fats

Pepsin inactivated by pH increase

Pancreatic enzymes

Question 2

Jejunum

Answer 2

Most nutrients absorption occurs here

Question 3

Ileum

Answer 3

Has peyer’s patch- aggregated lymphoid nodules

Question 4

Function of small intestine

Answer 4

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

Question 5

Small intestine surface area

Answer 5

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

Question 6

Crypt- villus unit

Answer 6

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

Question 7

Celiac sprue

Answer 7

Malabsortion syndrome caused by hypersensitivity to wheat, gluten and gliaden

Immune mediated destruction of the small intestinal villi

Question 8

Function of pancreas in digestion

Answer 8

Secretes about 1.5l of juices

Secretes alkaline fluid that neutralises chyme

Secretes enzymes that break down macromolecules in food

Question 9

Functional anatomy of panreas

Answer 9

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

Question 10

Diabetes

Answer 10

Pancreas does not make enough insulin or body cell’s do not use insulin effectively

Question 11

Composition of pancreatic juices

Answer 11

Proteolytic enzymes

Lipolytic enzymes

Amylolytic enzymes

Nucleases

Others

Question 12

Regulation of pancreatic secretions

Answer 12

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

Question 13

Mechanisms of enzyme secretions by acinar cells

Answer 13

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

Question 14

Prevention of pancreatic autodigestion

Answer 14

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

Question 15

Pancreatitis

Answer 15

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

Question 16

Mechanism of isotonic NaCl primary secretion by acinar cells

Answer 16

1. NaK pump creates inwardly directed Na+ gradient across basolateral membrane
2. Na/K/Cl cotransporter produces net Cl- uptake, driven by Na+ gradient, generated by NaK pump
3. Rise in intracellular K+ results from activity if pump and cotransporter is shunted by basolateral K+ channels for exit pathway for K+
4. Intracellular accumulation of Cl- makes electrochemical gradient driving Cl- secretion into acinar lumen through apical membrane Cl- channels
5. Movement of Cl- into lumen makes transepithelial voltage more lumen negative, driving Na+ into lumen via tight junction

Question 17

Secretions of pancreatic duct cells

Answer 17

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

Question 18

Mechanism of HCO3- secretion by ductal cells

Answer 18

1. HCO3- from cell cytoplasm into lumen via Cl-/HCO3- exchange in luminal cell membrane
2. Cl- recycled from lumen into cell via CFTR Cl- channel
3. Na+ secreted into duct lumen following HCO3- secretion, water follows by osmosis to make fluid secretion

Question 19

Ionic composition of pancreatic juice depends on rate of secretion

Answer 19

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-

Question 20

Dysfunction in ductal Cl- channel

Answer 20

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

Question 21

Bile functions

Answer 21

Provides alkali to neutralise acid

Provides bile salts to facilitate absorption of fats

Acts as vehicle for excretion of breakdown components

Question 22

Composition of bile

Answer 22

Water

Ions

Bilirubin and biliverdin

Bile salts

• synthesised from cholesterol
• required for emulsification and absorption of lipids, cholesterol and phospholipids

Question 23

Organisation of liver

Answer 23

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

Question 24

Gallbladder and bile

Answer 24

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

Question 25

Biliary tree

Answer 25

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

Question 26

Mechanisms of secretion

Answer 26

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

Question 27

Bile acid dependent fraction- bile salts

Answer 27

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

Question 28

Enterohepatic circulation regulates secretion of bile acids

Answer 28

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

Question 29

Bile pigments

Answer 29

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

Question 30

Hepatic jaundice

Answer 30

Accumulation of bilirubin in blood

• impaired uptake by hepatocytes
• failure to conjugate bilirubin
• as seen in hepatitis and cirrhosis

Question 31

Control of bile release into duodenum following meal

Answer 31

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

Question 32

Dysfunction of gall bladder

Answer 32

Ranges from cholelithiasis ( to biliary colic