5.1: Liver and Pancreas

Question 1

How is the acidic nature of the chyme entering the duodenum corrected for?

Answer 1

HCO3- is secreted by the pancreas, liver and duodenal mucosa (also protects the duodenum lining)

Question 2

How is the hypertonic nature of chyme entering the duodenum corrected for?

Answer 2

Osmotic movement of water across the duodenal wall

Question 3

Name two things that help to complete the digestion of chyme in the duodenum

Answer 3

1. Enzymes from the pancreas and SI mucosa

2. Bile acids from the liver (emulsify fats)

Question 4

What is gastric dumping and what is the resulting stool?

Answer 4

When the stomach empties its contents into the duodenum faster than normal, the resulting stool is explosive watery diarrhea

Question 5

What are the two main roles of the exocrine pancreas?

Answer 5

1. Neutralization; releases alkaline juice

2. Digestion; mix of enzymes known as proteases (digest proteins), amylase (sugars) and lipases

Question 6

Name four protease enzymes secreted by the exocrine pancreas

Answer 6

Trypsin, chymotrypsin, elastase, carboxypeptidase (Breaks off little chunks of proteins)

Question 7

Where in the exocrine pancreas are enzymes and alkaline juice secreted from?

Answer 7

Enzymes secreted from acini

Alkaline juice secreted from ducts; water and HCO3- ions modify the secretion on its way to the duodenum

Question 8

Describe in detail how the acinar secretion is formed and activated (in the pancreas)

Answer 8

1. Enzymes synthesized on ribosomes (mostly as inactive precursors)
2. Packaged into condensed vacuoles by Golgi
3. Form zymogen granules which are secreted by exocytosis
4. Zymogen granules are activated by enzymatic cleavage done in the duodenum (e.g; trypsinogen -> trypsin)

Question 9

How could pancreatitis occur and how would it affect the pancreas’ ability to secrete enzymes?

Answer 9

Can occur if there is inappropriate activation of pancreatic enzymes before they’ve reached the duodenum (i.e in the ducts)

In pancreatitis, enzymes are released into the blood, can test particularly for amylase

Question 10

What constitutes the intestinal phase of the pancreatic acinar secretion? *Include what stimulates the secretion

Answer 10

CCK stimulates the acinar secretion, and it is released by duodenal APUD cells when there is fat and hypertonicity in the intestine

Question 11

What also stimulates the CCK receptor (other than CCK) and why?

Answer 11

Gastrin, as CCK and gastrin are very similar in structure

Question 12

Describe the cephalic phase of the pancreatic acinar secretion

Answer 12

Vagus nerves stimulate the release of Ach

Question 13

How do ductal cells in the pancreas drive the secretion of HCO3- into the lumen?

Answer 13

[HCO3-] is elevated in the blood due to the gastric secretion of acid

Na+-K+ ATPase creates a low intracellular [Na+] and a higher [K+]. This allows for the Na+/H antiport to pump Na+ into the cell and H+ out to combine with HCO3- in the ECF and become H20 and CO2. They can then move back into the cell, rejoin and remake H+ and HCO3- in the cell. The resulting H+ is then put back out into the ECF and the HCO3- is pumped through the luminal membrane into the blood.

Question 14

What stimulates the pancreatic ductal secretion and where does it come from?

Answer 14

Secretin, released from jejunal cells in response to low pH. Their action is also facilitated by CCK (and acini secretions)

Question 15

What is the liver’s role when it comes to blood?

Answer 15

Produces plasma proteins, detoxifies the blood and plays a part in energy metabolism

Question 16

How much bile does the liver excrete per day? What other secretions does it provide for the gut?

Answer 16

0.25-1L/day. In addition to bile acids the liver secretes alkaline juice for digestion and bile pigments (like bilirubin, excreted as bile)

Question 17

Describe how blood and bile flows around the liver

Answer 17

75% of blood flows into the portal triad via the portal vein and 25% comes from the hepatic artery. Terminal branches of the portal vein and hepatic artery empty together and mix as they enter sinusoids in the liver. As blood flows through the sinusoids, a considerable amount of plasma is filtered into the space between hepatocytes and endothelia to become lymph. Blood then flows central into the Central canal which coalesces into the hepatic veins that lead to the IVC

Question 18

Describe the pathway of bile as it flows through the liver and into the SI.
*Include how this relates to the portal triad structure

Answer 18

Bile (produced by hepatocytes) first flows through canaliculi (dilated space between adjacent hepatocytes) parallel to blood in the sinusoids (lined with hepatocytes) but in the opposite direction towards the bile ducts (which begin in close proximity to the terminal ends of the hepatic artery and portal vein); a cluster of structures that form the portal triad. Small bile ducts anastomose into larger ones which then forms the common bile duct which dumps into the duodenum. During periods of time where bile is not dumped into the SI, it is diverted into the gall bladder (where it is stored) which feeds back into the common bile duct via the cystic duct

Question 19

Describe the bile acid ‘independent’ and ‘dependent’ components of bile, where is each component secreted from?

Answer 19

The bile acid dependent component contains bile acids and pigments that are secreted into canaliculi by hepatocytes

The bile acid independent components are alkaline juices secreted by duct cells (alike to pancreatic duct cell secretions).

Question 20

What is the major component of bile acids and how do they travel?

Answer 20

Bile acids are synthesized from cholesterol. The cholesterol is conjugated (attach a couple amino acids on the end) to make easier for transport, and travels in bile as micelles.

Question 21

What makes up the initial micelle and what is its function?

Answer 21

Majorly cholesterol but also bile acids and phospholipids, the initial micelles are carriers for the bile acids and pigments

Question 22

What happens when cholesterol cannot be conjugated for transport?

Answer 22

The result is unconjugated bilirubin that leads to neonatal jaundice

Question 23

Why are bile acids so essential in the digestion/absorption of fat?

Answer 23

They emulsify the large globules leaving the stomach (as stomach acid breaks down the natural emulsions) to provide a higher surface area for lipases to cleave fatty acids and glycerol

Question 24

What is the role of colipase?

Answer 24

Links bile acids and lipases and spreads them over the surface of the fatty globule

Question 25

What happens once fatty acids are cleaved from the emulsified globule? How do they travel and how are they initially absorbed?

Answer 25

They form another micelle, which has hydrophobic fatty acids within and polar groups of bile acids (need them to be above a critical concentration to form) on the outside which enables the molecule to travel. The micelles bring these hydrophobic molecules into an ‘unstirred layer’ (slower moving lumen, imagine the edges of a river) next to epithelia where fatty acids are released slowly and enter cells by diffusion.

Question 26

What happens once fatty acids diffuse into epithelia cells?

Answer 26

They re-synthesize lipids within the epithelia and are exported to lymphatics via chylomicrons

Question 27

What happens to the bile acids once fatty acids have been absorbed into the epithelia?

Answer 27

They are released into the lumen and continue to the terminal ileum. Here they are actively absorbed into the terminal ileum’s epithelium and return to the liver’s sinusoids via the hepatic portal vein. Hepatocytes actively take up the bile acids and re-secrete them into canaliculi

Question 28

What is the entero-hepatic circulation and what is its significance?

Answer 28

The flow of bile from the liver -> duodenum -> terminal ileum before re-entering the hepatic portal blood to travel back to the liver, this allows the bile acids to cycle

Question 29

If bile acids are constantly circulating, why must hepatocytes synthesize more?

Answer 29

Not all bile acids are recovered as some are unconjugated by bacterial action in the gut and lost.

Question 30

When do bile acids return to the liver?

Answer 30

Between meals

Question 31

How does the gall bladder accommodate to store large amounts of bile? What is the potential danger?

Answer 31

The gall bladder transports salt and water out of the bile acids (dehydrates it), this can overly concentrate the bile acids which increases the risk of precipitation and gall stones

Question 32

What stimulates the gall bladder to release bile into the duodenum? What immediately happens once it reaches the duodenum?

Answer 32

Contraction of the gall bladder muscle is stimulated by CCK (secreted by duodenum in response to gastric emptying). The concentrated bile acids are ejected together with enzymes from the pancreas and alkaline juices (from the pancreas and liver)

Question 33

What is steatorrhoea and why does it occur?

*Provide three descriptive adjectives

Answer 33

Steatorrhoea occurs when bile acids or pancreatic enzymes are not secreted in adequate amounts and so fat remains undigested and is excreted in feces. The resulting feces are pale, floating and foul smelling

Question 34

Describe the production and excretion process of bile pigments and what happens when the process goes wrong

Answer 34

Production: cholesterol is converted into the bile acids (cholic and chenodeoxycholic acid) which are conjugated to an amino acid to yield the conjugated form. Their amphipathic nature allows them to emulsify fats and solubilize them within a micelle so they can be transported

1. Hb is separated into heme and globin (by kupffer cells, the spleen or lymph node macrophages)
2. Heme is transferred to the bone marrow, leaving behind biliverdin which is reduced to bilirubin (a yellow compound)
3. Bilirubin goes to the liver as a substrate for bile production, once bile has done its job the pigment is excreted as
   A) stercobilin in feces (brown)
   B) urrobilin in urine (straw-coloured)

If bile pigments are unable to be excreted properly it accumulates in the blood causing jaundice

Question 35

Which structure helps suspend the liver within the abdomen?

Answer 35

Hepatic veins which join to the IVC and anterior abdominal wall muscles

Question 36

Define the terms hepatic lobule and hepatic acinus

Answer 36

Hepatic lobule: each has six portal triads at its periphery and a central vein at its central axis

Hepatic acinus: spans 2 portal triads and a long axis defined by a line drawn between 2 central veins that forms the outermost pole of the acinus

Question 37

What are Kupffer cells and what is their role in the body?

Answer 37

Specialized macrophages in the walls of hepatic sinusoids:

1. Promote normal liver physiology
2. Release inflammatory mediators in response to toxic compounds
3. Role in the reticuloendothelial system