W10.1_Liver Physiology

Question 1

State the related organs to liver. Describe how the liver is separated into lobes or segments. Explain the characteristics of the liver in terms of its blood volume and supply.

Answer 1

• Related organs: liver, gallbladder, pancreas, bile passage
• Two lobes (L/R) separated by midline (falciform ligament)
• Subdivided into nine functional segments based on vascular supply and biliary drainage
• Receives disproportionally high blood volume
  (2% body weight vs 20% cardiac output)
• Dual blood supply of splanchnic blood (nutrient rich and deoxygenated) from gut via hepatic portal vein and oxygenated blood from hepatic artery to sustain hepatocytes

Question 2

What are liver composed of? Explain how they receive blood and its blood flow. State the function and special feature of sinusoids.

Answer 2

• Composed of small lobules
• Lobule is surrounded by branches of hepatic artery and portal vein (to receive oxygen and nutrients)
• Vessels drain into sinusoids (capillary-like structures) -> drain to a central vein (deoxygenated blood in hepatic vein)
• Sinusoids conduct material exchange (similar to capillaries)
• Incomplete surrounding diaphragm (basement membrane) and large intercellular gaps in endothelial layer causes them to have increased permeability -> larger molecules/proteins to cross

Question 3

State the properties and functions of different hepatic cells (5).

Answer 3

• Sinusoidal endothelial cells: form sinusoidal blood vessels, less tight junctions
• Kupffer cells: macrophages of liver, reside in lumen of sinusoids, exposed to immunogens from gut
• Stellate cells: in perisinusoidal space, in quiescent state/activated in response to damage to repair
• Hepatocytes: large, cuboid, extremely metabolically active, large numbers of smooth ER, binucleate, contains bile canaliculus that starts production of bile
• Cholangiocytes: epithelial cells lining the bile ducts

Question 4

Explain how liver process nutrients. Discuss it in terms of proteins, carbohydrates, and fats.

Answer 4

• Metabolism of nutrients absorbed and transported from small intestine
• Convert into forms that can be stored (glycogen/fat/iron/vitamin A) or used
• Mediates transport to other tissues
• Proteins: break down and excrete excess proteins (as body cannot store amino acids) by removing toxic amine group through deamination and converting into harmless urea -> excreted by kidneys
• Synthesising of non-essential amino acids is also possible (transamination)
• Carbohydrates: excess glucose stored as glycogen/breaks down glycogen into glucose and exports it, synthesise from fats/proteins when hepatic glycogen reserves is exhausted, coordinated by pancreatic hormone (insulin, glucagon)
• Fats: synthesis of phospholipids and cholesterol, converting excess carbohydrates/proteins into fat, surplus cholesterol is converted into bile salts and excreted

Question 5

Explain the functions of VLDL, LDL, and HDL.

Answer 5

• VLDL (very low density lipoprotein): allows triglycerides made in liver to be transported
• LDL: transports cholesterol to cells -> raises blood cholesterol levels
• HDL: transports excess cholesterol from cells back to liver -> lowers blood cholesterol cells

Question 6

Explain how vitamin A is stored in liver and used by some immune cells.

Answer 6

• Vitamin A/retinol absorbed from gut -> carried in blood in a complex with TTR -> stored in stellate cells
• Some immune cells (dendritic cells) convert retinol to retinoic acid -> very metabolically active to deliver to cells that need it -> metabolised by liver again

Question 7

What are the toxins that can be processed in the liver? How does the detoxification in liver work?

Answer 7

• Pollutants, pesticides, drugs, alcohol, microorganisms…
• Liver converts them into less harmful/more hydrophilic forms -> excretion
• Through phase 1 and 2 metabolism routes as well

Question 8

Briefly explain the production of proteins in liver and each of their functions.

Answer 8

• (except immunoglobulins)
• ex. albumins (regulate osmotic pressure in blood), globulins (transport and antibodies), fibrinogens (part of blood clotting cascade to form fibrin clot)
• Made in hepatocytes (ER, golgi apparatus)

Question 9

Explain how bile is produced in liver and secreted into the small intestine. What is the function of gallbladder? How is bile recylced in the body?

Answer 9

• Stimulated by chyme -> neutralise acidic environment in stomach to favour intestinal enzymes
• Produced in liver (bile acids secreted by hepatocytes, cholangiocytes modify bile volume and composition) -> stored in gallbladder -> secreted into small intestine to emulsify fats
• Gallbladder: storage of bile, absorption of water to concentrate bile, release to duodenum when responds to cholecystokinin (CCK released from duodenum) by dilating and contracting through pressure control
• Bile is recycled through hepatic portal vein -> bile salts stimulate hepatocytes to secrete more