6. Chyme pancreas & liver

Question 1

describe and explain the osmotic and pH status of chyme as it enters duodenum

Answer 1

1. hypertonic:
   
   • initial digestion of food creates solutes
   • stomach wall largely impermeable to water so water cannot move in to dilute chyme
2. acidic: low pH due to gastric HCl secretion

Question 2

why must chyme release from stomach be regulated

Answer 2

Duodenum is relatively permeable to water so hypertonic chyme draws in water from ECF/circulation and becomes isotonic.

If chyme release not controlled, risk of overwhelming duodenum - would cause massive pull of water from ECF causing decreased blood volume and BP

Question 3

which hormones does the duodenum release in response to chyme presence and what is their effect

Answer 3

Duodenum secretes into blood:

1. SECRETIN (in response to low chyme pH)
   - stimulates pancreas duct cells to release aqueous HCO3- component of pancreatic secretion… increases pH of chyme
2. CCK (in response to hypertonicity, small peptides and fats in chyme)
   - stimulates pancreas acini to secrete digestive enzymes… continues digestion
   - stimulates gallbladder contraction and sphincter of Oddi relaxation… bile release… continues digestion

Question 4

why enzymes are released by pancreatic acini

Answer 4

1. amylases (active)
2. lipases (active)
3. proteases (inactive) - only activated inside duodenum (by low pH and enteropeptidase) to prevent pancreatic auto-digestion

Question 5

which factors stimulate pancreatic exocrine secretion

Answer 5

1. duodenal hormones: secretin and CCK

2. autonomics: sympathetics inihibit, parasympathetics (vagus n.) stimulate

Question 6

describe the arterial supply to the pancreas

Answer 6

• branches of splenic a.

- branches of superior and inferior pancreatico-duodenal aa. (from gastroduodenal and superior mesenteric aa.)

Question 7

what is the main function of liver in digestion

Answer 7

secretes BILE (stored in liver) into duodenum - key role in emulsifying lipids so that they can be readily digested by pancreatic lipases

Question 8

hepatocytes in which acinar zones are more susceptible to toxic and ischaemic damage

Answer 8

Z1 - toxic

Z3 - ischaemic

Question 9

what are the 2 main components of bile

Answer 9

1. alkaline juices - secreted by bile duct cells
2. bile acids and pigments - hepatocytes produce bile acids which are then conjugated with amino acids (eg glycine) to form bile salts (as acids aren’t always soluble at duodenal pHs - functionally useless)

Question 10

what is the role of bile salts in lipid digestion

Answer 10

1. bile salts (amphipathic structure allowing them to act at oil/water inferface) emulsify lipids into smaller units/droplets to increase surface area (instead of large globules)…
2. allows lipases to catabolise lipids…
3. bile salts then create micelles with products of lipid breakdown (cholesterol, monoglyerides, free fatty acids) to transport these towards enterocytes…
4. lipids then diffuse into enterocytes whilst bile salts remain in gut

Question 11

what happens to bile acids after having transported lipid products

Answer 11

are reabsorbed in terminal ileum and returned to liver in portal blood (entero-hepatic circulation) for recycling

Question 12

what happens to lipid molecules inside enterocytes

Answer 12

i. are re-esterified back to TAGs, phospholipids and cholesterol, and packed with apoproteins to create chylomicrons…
ii. chylomicrons exocytosed from basolateral membrane into lacteals… enter lymphatic capillaries

Question 13

what is steatorrhoea

Answer 13

appearance of undigested fat in faeces (pale, floating, foul smelling) as a result of inadequate secretion of bile acids/salts or pancreatic lipases