012 Physiology of the GI tract: Pancreas and Liver

Question 1

How much volume does the pancreas secrete each day?

Answer 1

1-1.5L

Question 2

What is the tonicity and pH of pancreatic secretion?

Answer 2

pH 8, isotonic

Question 3

What enzymes does the pancreas secrete?

Answer 3

Lipase, Amlylase, Ribunuclease/ deoxyribonuclease, carboxypetidase, trypsin, chymotrypsin

Question 4

What is located in the endocrine portion of the pancreas and what does it secrete?

Answer 4

The islets of langerhans are found in the pancreas, secreting insulin and glucagon to regulate blood glucose.

Question 5

Why does the pancreas secrete inactive enzymes?

Answer 5

If they are active when secreted, it will attack the pancreas itself. Hence, it is only activated in the upper small intestines.

Question 6

Explain the nervous control of pancreatic secretion.

Answer 6

Vagus nerves from the parasympathetic nervous system release ACh neurotransmitters that activate gastrin release from G-cells. This increases acinar cell secretion. The nerves also directly activate acinar and duct cell secretion.

Question 7

What is the function of CFTR? What happens if this is not present? (Including cystic fibrosis)

Answer 7

Cl- is normally secreted into the ductal lumen via the cystic fibrosis transmembrane regulator (CFTR). This provides a driving force for fluid movement necessary to maintain solubility of ductal enzymes. Impaired Cl- transport causes low volumes of more acidic fluid, causing enzyme precipitation. This obstruction of ducts (cystic fibrosis) causes progressive pancreatic damage and reduced efficiency of nutrient digestion.

Question 8

Explain the 3 functions of bile.

Answer 8

Emulsifies fats to increase efficiency of lipase.
Activate pancreatic lipase and cholesterol esterase.
Micelle formation to convert fat soluble compounds in solution rather than in suspension.
Excretion of metabolic waste.

Question 9

What is the difference between a micellar solution and an emulsification?

Answer 9

A micellar solution contains much smaller particles with greater stability

Question 10

Describe the structure of a micelle, including what it is able to absorb.

Answer 10

Micelles have hydrophilic shells containing a hydrophobic core that contains cholesterol. It is able to absorb fat-soluble vitamins, cholesterol, fatty acids and monoglycerides.

Question 11

What is the purpose of pancreatic amylase?

Answer 11

Converting polysaccharides into glucose and maltose

Question 12

Explain the formation of a micelle in the duodenum from the ingestion of fat.

Answer 12

When fat globules enter the duodenum, it interacts with bile salts. These bind to the fats, which repel eachother and break them up into stable emulsion in solution. This allows lipase to break them into fatty acids and monoglycerides. Some monoglycerides are able to be absorbed in epithelia, however, most cannot cross the epithelia due to water barrier on the brush border membrane. Instead the rest form mixed micelles (structures formed in the liver). These micelles are formed from 20-40 bile salt molecules and lecithin.

Question 13

Outline the steps in the formation of bile and secretion into the duodenum

Answer 13

Cholesterol forms bile acid in the hepatocytes. This is converted into bile acids, which are then conjugated with glycine/taurine to form bile salts. Bile salts are pumped into the hepatocyte through the Na+/BS- symporter. Na+ gradient is achieved by the primary active transport of K+/Na+ ATPase, as Na+ is moved out of the hepatocyte for K+ moved in. Na+/BS- symporter acts through secondary active transport. Bile salts then enter the canaliculus, with the addition of H2O and Na+.
When acid reaches the duodenum, secretin activates CFTR. These transporters allow for the secretion of Cl- into the bile duct. This activation of CFTR then stimulates the Na+/HCO3- co-transporter on the basal lateral membrane, moving bicarbonate into the cells.
The Cl- in the lumen of the bile duct then drives Cl-/HCO3- antiporter to exchange HCO3- out of the apical part of the cell for Cl- moved back in. This also forces water (osmotic balance) and Na+ to be secreted paracellularly.

Question 14

What happens when red blood cells are broken down?

Answer 14

When RBC are broken down, they are engulfed by macrophages. The haemoglobin is broken down into bilirubin, which is conjugated in the liver and excreted in the bile.

Question 15

Explain the hormonal control of pancreatic secretion (hint: CCK, secretin, gastrin). Remember to include the stimulus, the cells producing them and their effect.

Answer 15

During the cephalic and gastric phase, the stimulation of vagal nerves cause gastrin release from G-cells, and also result in acinar and duct secretion, releasing enzymes from zymogen granules. During the intestinal phase, when acidic chyme is delivered into the duodenum, the decrease in H+ (pH<3.5) will cause secretin release and presence of fat and protein will cause CCK release by enteroendocrine cells. This enters the blood and feeds back to the pancreas to secrete bicarbonate enzymes and digestive enzymes respectively.

Question 16

List a few functions of the liver

Answer 16

Protein metabolism
Carbohydrate metabolism
Lipid metabolism
Iron storage 
Detoxification
Synthesis and secretion of bile

Question 17

Explain the movement of ions in and out of the duct as it moves along the intracellular ductal system to extralobular ductal system.

Answer 17

Along the intralobular ductal system, Na+, K+, Cl- and HCO3- are transferred from plasma to the primary secretions. As it moves along the extralobular system, HCO3- and H2O are further secreted in response to an absorption of Cl-.

Question 18

How much of pancreatic secretion occurs in the 3 phases?

Answer 18

Cephalic phase - 20%
Gastric phase - 10%
Intestinal phase - 70%

Question 19

What happens if there is reduced excretion of bile salts?

Answer 19

Reduced excretion of bile may lead to accumulation of bile pigments in the extracellular fluid, resulting in jaundice.

Question 20

How much hepatic bile does the gall bladder store during interdigestive phase?

Answer 20

50%

Question 21

How does the gall bladder concentrate hepatic bile?

Answer 21

It concentrates bile by 20% by absorbing Cl-, HCO3-, H20 and Na+, leaving a critical micelle concentration.

Question 22

Which hormone causes gall bladder concentration

Answer 22

CCK in the presence of chyme.

Question 23

What is the pH and tonicity of bile?

Answer 23

Isotonic at alkaline pH of 7.6-8.6

Question 24

What does bile contain?

Answer 24

Cholesterol, bile salts and acids, water and electrolytes, bilirubin

Question 25

How much bile do we ingest per day, and how much bile do circulates in our body per day?

Answer 25

0.3-0.5g of bile acid per day
3g circulating pool
total of 30g secreted per day

Question 26

What is the difference between bile acids and bile salts?

Answer 26

Bile acids are water-soluble derivatives of cholesterol synthesized in the liver. Bile salts are conjugates of bile acids and amino acids (taurine or glycine) before their secretion of bile.

Question 27

How does the gall bladder modify the bile acid?

Answer 27

Gall bladder has very tall hepatocytes with large lateral channels that are able to generate very high osmotic gradients within lateral area of the cells. This gradient draws in water and electrolytes (HCO3-, Cl-, Na+, H2O) into the cell and across the cell, reaching critical micelle concentration.

Question 28

How does the parasympathetic system regulate bile acid secretion?

Answer 28

Parasympathetic system increases blood flow to the liver and delivers more bile acids back to the hepatocytes for bile secretion.

Question 29

How does a micelle help in the absorption of certain substances?

Answer 29

Hydrophilic shell allows the micelle to diffuse into the unstirred water layer of epithelial surface. As it reaches jejunal epithelial surface, it releases fatty acids and monoglycerides. The micelle themselves do not get absorbed but continue moving down small intestine to pick up.

Question 30

What happens after monoglycerides and fatty acids are absorbed in enterocytes?

Answer 30

They are delivered to smooth endoplasmic reticulum where they are converted back to triglycerides, phospholipids and cholesterol esters. These are encased by chylomicron, which fuse with hepatocyte basal lateral layer endothelium to undergo exocytosis into the lacteals.

Question 31

What happens after monoglycerides and fatty acids are absorbed in enterocytes?

Answer 31

They are delivered to smooth endoplasmic reticulum where they are converted back to triglycerides, phospholipids and cholesterol esters. These are encased by chylomicron, which fuse with hepatocyte basal lateral layer endothelium to undergo exocytosis into the lacteals.

Question 32

Describe entero-hepatic circulation of bile salts

Answer 32

Bile acids move across lower ileum into the hepatic portal vein. Hepatocytes then take these up through a Na+ dependant process and are resecreted into the bile.