Session 6 - Pancreas and Liver

Question 1

What are the characteristics of chyme when it leaves the stomach?

Answer 1

Low pH
Hypertonic (> substance breakdown, so > solutes)
Partially digested

Question 2

How does the duodenum protect itself against the low pH of the chyme from the stomach?

Answer 2

Brunners glands - secrete alkaline mucus.

Question 3

How does tonicity of chyme change as it moves through the duodenum?

Answer 3

Hypertonic chyme draws movement of water from ECF into duodenum, as the duodenum is more permeable to water.
The chyme should be isotonic by the time it leaves the duodenum.

Question 4

How is digestion in the duodenum hormonally controlled?

Answer 4

Secretin and CCK (cholecystokinin)
- both secreted by duodenum in response to chyme.

Secretin
- stimulates the pancreas to release aqueous bicarbonate part of pancreatic secretions.

CCK

• stimulates pancrease to release enzyme component.
• stimulates gall bladder to contract and sphincter of oddi to relax.

Question 5

How are pancreatic secretions controlled?

Answer 5

Hormonally - CCK and Secretin
Neurally - Autonomics
- sympathetic inhibits, parasympathetic stimulates.

Question 6

How are the cells of the pancreas structured?

Answer 6

• Acini: secrete components into ducts.

- Acinar cells make up acinus, and duct cells form ducts.

Question 7

Describe the stages of pancreatic secretions.

Answer 7

• CCK (+vagus) stimulate acini to produce enzymes.
  (Amylases + Lipases - active, proteases - inactive)
• Secretin stimulates bicarbonate aqueous component to be secreted from duct cells.

Question 8

In what form are proteases released from the pancreas, why is this important?

Answer 8

Released as inactive zymogens - which are activated in the duodenum lumen. To prevent them self digesting the pancreas.

Question 9

Why does a blocked pancreatic duct/ ampulla of vater cause pancreatitis?

Answer 9

• Proteases build up in pancreas and become activated, and self digest pancreas, causing inflammation and damage.
  (most commonly gallstone)

Question 10

Where is bile made, what does it contain?

Answer 10

• Secreted by the liver.

- Consists of bile acids, bile pigments, and alkaline solution.

Question 11

What is the role of bile?

Answer 11

It emulsifies lipids in the duodenum, so they can be readily digested by lipases.

Question 12

How is the liver structured?

Answer 12

Formed from Lobules - the structural units of the liver.

A hexagonal arrangment of cells, bile ducts and blood vessels.

Question 13

Describe the structure of a liver lobule.

Answer 13

Outer triad of structures:

1) Portal vein branch
2) Hepatic artery branch
3) Bile duct

A Central vein in the middle.

Blood flows in towards middle vein, through sinusoids.
Bile flows out along canaliculi into the bile duct.

Question 14

How is the lobule separated into zones? Which parts will be damaged by different things?

Answer 14

• Functional area is called acinus.
• Split into zones 1, 2 and 3.
• 1 Closest to artery, 3 furthest away.

Toxic damage most likely in zone 1, whereas ischaemic damage most likely in zone 3.

Question 15

How is bile formed and stored?

Answer 15

Formed by hepatocytes and duct cells in liver, continuously produced.
Stored in gall bladder- where it is concentrated.

Question 16

Where do the two main components of bile come from?

Answer 16

• Bile acids/ pigments - hepatocytes

- Alkaline juices - bile duct cells (stimulated by secretin)

Question 17

What are bile salts?

Answer 17

Conjugated bile acids!
Two primary acids: Cholic acid, Chenodeoxycholic acid.

They are conjugated with amino acids (glycine, taurine)

Question 18

Why are bile acids conjugated?

Answer 18

Because they are not always soluble at duodenal pH, which would make them non functional.
Bile salts have amphipathic structure:
- Hydrophilic end, and hydrophobic end.

Allows emulsification of dietary lipids.

Question 19

How do bile salts aid in the digestion of lipids?

Answer 19

• They emulsify the large lipid globules into smaller units, increasing the surface area for lipases to work.
• They also create micelles with the products of lipid breakdown, which act as a vehicle for transporting hydrophobic molecules towards enterocyte.

Question 20

How do micelles help transport products of lipid digestion? What happens to the bile salts?

Answer 20

Micelle transports digested lipid to luminal membrane of enterocyte.
Lipids diffuse into epithelial cell.

The bile salts are reabsorbed in the terminal ileum, and returned to liver in portal blood for recycling (back into bile acids)

Question 21

How are digested fats transported into the circulation?

Answer 21

• In enterocyte are re-esterified into TAG, phospholipids and cholesterol.
• Form chylomicrons containing many apolipoproteins.
• Exocytosed into lymphatic capillaries (too big for capillaries), and travel to thoracic duct where they enter the blood.

Question 22

What is steatorrhoea?

Answer 22

Undigested fat in the faeces.
- Pale, Floating, Fould smell.

Caused by not enough secretion of bile acids or pancreatic lipases.