Pancreas and liver

Question 1

Describe chyme as it enters the duodenum from the stomach

Answer 1

• Hypertonic
• Hypertonicity increases as digestion takes place
• Acidic
• Partially digested

Question 2

What mechanisms are used by the GI system to make chyme more suitable for the duodenum?

Answer 2

• Water is added to chyme from ECF or circulation to make it isotonic
• Enzymes + bicarbonate ions secreted from pancreas
• Bicarbonate ions + bile secreted from liver

Question 3

Why is chyme hypertonic?

Answer 3

• Food produces lots of solutes that are dissolved in gastric juice
• Stomach is largely impermeable to water
• Water cannot dilute solute in chyme

Question 4

Outline how water is added to chyme in the duodenum

Answer 4

• Duodenum is relatively permeable to water
• Introduction of hypertonic chyme draws water into duodenum from extracellular fluid
• Pyloric sphincter controls how much chyme enters duodenum so it does not get overwhelmed
• Chyme is isotonic when it leaves duodenum

Question 5

What is the function of Brunner’s glands?

Answer 5

• Secrete alkaline mucus to protect epithelium of duodenum from acidic chyme

Question 6

How much of the pancreas is exocrine?

Answer 6

• 90%

Question 7

What is the basic structure of the pancreas?

Answer 7

• Acinus produces enzymes
• Centroacinar cells produce aqueous component of secretions
• Terminal duct modifies aqueous secretions
• Terminal ducts join to form major pancreatic duct
• Major pancreatic duct releases secretions into duodenum when sphincter of Oddi is relaxed

Question 8

Outline the innervation of the pancreas

Answer 8

• Sympathetic nervous system inhibits exocrine function by reducing blood supply
• Vagus nerve (parasympathetic) stimulates
• Also regulated by hormones

Question 9

What stimulates an acinus to release enzymes?

Answer 9

• Vagus nerve and cholecystokinin (secreted by I cells)
• These are stimulated when hypertonicity/small peptides/ fats are detected in the duodenum

Question 10

Which enzymes are produced by acini?

Answer 10

• Amylases/lipases (active)
• Proteases (inactive) e.g. trypsin, chymotrypsin, elastase, carboxypeptidase

Question 11

Outline how enzymes are produced and packaged in an acinus

Answer 11

• Formed on rough endoplasmic reticulum
• Moved to Golgi complex
• Condensing vacuoles
• Concentrated in zymogen granules
• Released with appropriate stimulus (parasympathetic or CCK)

Question 12

Describe zymogen granules

Answer 12

• Membrane bound
• Contain zymogen - an inactive pre-cursor of an enzyme e.g. trypsinogen
• Prevents pancreas being auto-digested

Question 13

How can we tell if the pancreas has been damaged?

Answer 13

• If pancreatic enzymes appear in blood

Question 14

What are the functions of the liver?

Answer 14

• Energy metabolism
• Detoxification - everything absorbed from gut drains through liver
• Plasma protein production
• Secretes bile into gut

Question 15

What are hepatocytes?

Answer 15

• Chief functional cell of liver
• Surround vasculature and bile architecture
• Compromise 80% of mass of liver
• Very active at producing proteins/lipids for export

Question 16

How are hepatocytes adapted to their function?

Answer 16

• Contain lots of rough/smooth ER
• Contain lots of stacks of Golgi membranes
• Contain lots of glycogen

Question 17

What is meant by the statement that the liver has a dual blood supply?

Answer 17

• Liver has blood supply from hepatic artery and hepatic portal vein
• Artery gives 20% of blood
• Portal vein gives 80% of blood
• Portal vein blood is not as deoxygenated as normal venous blood, so liver obtains O2 from portal vein
• Venous and arterial blood mixes in sinusoids

Question 18

Describe the structure of a liver lobule

Answer 18

• Hexagon shaped
• Hepatic triad (bile duct, hepatic artery, portal vein) in each corner
• Central vein in middle

Question 19

Outline the venous drainage of a liver lobule

Answer 19

• Blood travels from portal vein to central vein
• Central veins merge and drain into hepatic vein
• 3x hepatic veins drain into inferior vena cava

Question 20

Outline the structure of an acinus

Answer 20

• Region where liver lobules join
• Diamond-shaped
• Has 3 zones
• Zone 1 in closest proximity to portal triads so any toxin coming into liver will affect zone 1 the most
• Zone 3 is closest to central vein so is most likely to be affected by ischaemia

Question 21

How does bile leave the liver?

Answer 21

• Flows along canaliculi until it reaches bile ducts
• Then flows into duodenum when sphincter of Oddi is relaxed

Question 22

What are the 2 components of bile?

Answer 22

1. Bile acid dependent
   - Secreted into canaliculi by hepatocytes
   - Contains bile acids and pigments
2. Bile acid independent
   - Secreted by duct cells
   - Similar alkaline solution to pancreatic duct cells
   - Stimulated by secretin

Bile also contains cholesterol

Question 23

What are the primary bile acids?

Answer 23

1. Cholic acid
2. Chenodeoxycholic acid
   - Further bile acids are formed in the gut

Question 24

What are bile salts?

Answer 24

• Bile acids that are conjugated with the amino acids glycine and taurine

Question 25

Why do we need bile salts in addition to bile acids?

Answer 25

• Bile acids are not always soluble at duodenal pHs whereas bile salts generally are
• Bile salts have an amphipathic structure which a hydrophilic end and a hydrophobic end
• They can therefore act at the oil/water interface
• This is crucial for the emulsification of dietary lipids

Question 26

How does bile help us digest fats?

Answer 26

• Lipids tend to form large globules by the time they reach the duodenum
• This creates a low surface area over which enzymes can act
• Bile acids emulsify fats into smaller units
• Helps disperse droplets
• Creates a larger SA for lipases to act on
• Bile acids then form micelles with products of lipid breakdown

Question 27

What are micelles?

Answer 27

• Grouping of bile acids surrounding breakdown products of fat
• Allows transport of hydrophobic molecules (cholesterol, monoglycerides, free fatty acids) through an aqueous environment
• Micelles diffuse with products to brush border of epithelial cells

Question 28

What happens once lipids have entered the intestinal epithelial cells?

Answer 28

• They are re-esterified back to triglycerides, phospholipids and cholesterol
• Then re-packaged with apoproteins into chylomicrons
• These transport fatty products around the body

Question 29

How do chylomicrons enter circulation?

Answer 29

• Exocytosis from basolateral membrane of intestinal epithelial cells
• Enter lymph capillaries (too large to enter normal capillaries)
• Travel through lymphatic system
• Re-enter vascular circulation at thoracic duct

Question 30

Outline the enterohepatic circulation of bile acids

Answer 30

• Bile salts remain in gut lumen
• Reabsorbed in terminal ileum
• Return to liver in portal blood
• Liver extracts bile salts and can then reuse them (instead of synthesising more)

Question 31

Where is the fundus of the gallbladder found?

Answer 31

• In transpyloric plane at level of L1

Question 32

What is the function of the gallbladder?

Answer 32

• Stores bile
• Concentrates bile by removing water/ions
• This can lead to gallstones

Question 33

What stimulates gallbladder contraction?

Answer 33

• CCK released from duodenum
• Relaxes sphincter of Oddi

Question 34

What is steatorrhoea?

Answer 34

• If bile salts or pancreatic lipases are not secreted in adequate amounts, fat appears in faeces
• Faeces are pale, floating and foul smelling

Question 35

Which components of bile are excreted?

Answer 35

• Bilirubin
• A breakdown product of haemoglobin
• Conjugated in liver
• Secreted into bile
• Excreted in faeces
• Otherwise accumulates in blood and causes jaundice