Functions of the Liver

Describe the storage, synthetic, metabolic and excretory functions of the liver

Storage

The liver is important in storage and release of:

• Carbohydrates as glycogen
  The adult liver stores ~100g of glycogen.
• Fat as triglycerides
• All fat-soluble vitamins (A, D, E, K)
• Many water soluble vitamins including folic acid and B₁₂
• Iron
• Copper

Synthetic

Synthetic functions include:

• Bile production
• Plasma proteins including:
  • Clotting factors
  • Albumin production
    120-300mg.kg^-1 of albumin is produced per day, dependent on nutritional status, plasma oncotic pressure, and endocrine function.

Metabolic

Metabolic functions include:

• Carbohydrate
• Fat
• Protein
• Bilirubin metabolism
• Drugs and Toxins

Carbohydrates

• Monosaccharides and disaccharides passively diffuse into hepatocytes
  Gradient is maintained by converting glucose to glucose-6-phosphate which is used to produce glycogen. This maintains the gradient for diffusion.
• Glycogen is either synthesised (glycogenesis) or broken down (glycogenolysis) depending on plasma glucose and insulin:
  • Increased blood glucose stimulates insulin release, increasing the formation of glycogen through activation of glycogen synthetase
  • Decreased blood glucose stimulates glycogenolysis and gluconeogenesis from amino acids.

Lipids

• Fat can be:
  • Stored as triglycerides
  • Hydrolysed to glycerol and fatty acids, which is used for ATP production

Proteins and Urea

Amino acids are absorbed from blood to be used for gluconeogenesis and for protein synthesis. In order to produce substrates for the CAC, Amino acids may be:

• Transaminated
• Deaminated
• Decarboxylated

The nitrogenous scrap of these reactions is urea, which is produced in several stages:

• A variety of metabolic processes convert amino acids to glutamate
• Glutamate is converted to ammonia by glutamate dehydrogenase
• Ammonia then enters the urea cycle to produce (surprisingly) urea, at the cost of 3 ATP
  • A normal diet of 100g protein per day produces ~30g of urea, and 1000mmol of hydrogen ions

Endocrine

• Produces angiotensinogen
• Produces IGF-1
• Converts T4 to T3

Immunoprotective

• Kupffer cells
  Tissue macrophages of the hepatic reticuloendothelial system. They phagocytose harmful substances including:
  • Endotoxins
  • Bacteria
  • Viruses
  • Immune complexes
  • Thrombin
  • Fibrin complexes
  • Tumour cells

Acid-Base Balance

May produce or consume large numbers of hydrogen ions:

• Carbon dioxide production
• Metabolism of organic acid anions
  • Lactate
  • Ketones
  • Amino acids
• Ammonium
• Production of plasma proteins
  Notably albumin

References

1. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
2. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
3. Brandis K. The Physiology Viva: Questions & Answers. 2003.