This chapter tries to summarise one of the main objectives of Section N1 from the 2017 CICM Primary Syllabus, which expects the trainee to "describe the storage, synthetic, metabolic, immunological and excretory functions of the liver." In historical exam papers the college has often asked for all of these to be "outlined" or "listed" in the space of a ten-minute SAQ answer:
"Testing breadth of knowledge rather than depth" was apparently the objective of these. What follows is therefore a superficial listing of the functions of the liver, grouped as logically as was afforded by the limitations of the famously logic-averse author. In case the reader values depth of knowledge as much as breadth, they can be referred to Kuntz & Kuntz (2008) who answer the CICM SAQ over about 40 pages of densely packed text. For a much shorter alternative, the functions of the liver are summarised in a table from this review article by Ozougwu et al (2017)
The way the college have phrased their syllabus entry is not conducive to learning. Consider: when discussing the storage functions of the liver, one would come across glycogen. Then, when discussing the synthetic function of the liver, one would once again trip over glycogen (which is synthesised in the liver). When thinking about the metabolic functions of the liver, one would inevitably have to mention glycogen as a major source of hepatic glucose. In short, it must have seemed sensible to divide the functions of the liver into storage, synthesis, metabolism, etc, but conceptually it would probably be better to explain it in terms of its functional roles. In that way, everything pertaining to carbohydrate metabolism would be together, and more easily understood. Still: the syllabus document is what it is. What follows is an attempt to squish a massive topic into a tiny space. Links of the main headings point to a chapter where each domain is thoroughly discussed.
- Storage functions of the liver:
- Metabolic fuel storage: glycogen (75g, 400kcal) and fat (75g, 600 kcal)
- Vitamin storage: fat soluble A, D, E and K, as well as B12
- Trace element storage: iron (as ferritin), copper, sink, selenium
- Blood reservoir function: contains up to 27% of total blood volume
- Synthetic functions of the liver:
- Synthesis of 90% of blood proteins (albumin, clotting factors, complement)
- Synthesis of regulatory molecules (thrombopoietin, angiotensinogen)
- Synthesis of nutrients: glucose (gluconeogenesis), ketones, lipids (VLDL), cholesterol, non-essential amino acids
- Synthesis of bile acids from cholesterol and urea from ammonia
- Synthesis of bilirubin (by Kupffer cells)
- Metabolic functions of the liver:
- Carbohydrate metabolism: conversion to and from glycogen
- Lipid metabolism: transformation into ketones or triglycerides
- Protein metabolism: deamination or transamination of amino acids
- Ammonia metabolism (into urea) and lactate metabolism (into glucose)
- Immunological functions of the liver:
- Synthesis of complement proteins (innate immunity)
- Regulation of protein synthesis to hinder infection (eg. restriction of systemic iron carriage and reduced production of negative acute phase reactants)
- Filtration of antigens from the blood by Kupffer cells and sinusoidal endothelial cells
- Excretory functions of the liver:
- Excrretion of bile acids (though 95% undergo reabsorption)
- Excretion of cholesterol in the bile (800-1200mg/day)
- Excretion of conjugated bilirubin in the bile
- Excretion of some ions (mainly sodium)
- Biliary excretion of drugs (ceftriaxone, apixaban, digoxin)
- Biliary excretion of heavy metals (lead, arsenic, mercury, cadmium)
Kuntz, Erwin, and Hans-Dieter Kuntz. "Biochemistry and functions of the liver." Hepatology Textbook and Atlas: History· Morphology Biochemistry· Diagnostics Clinic· Therapy (2008): 35-76.
Ozougwu, Jevas C. "Physiology of the liver." International Journal of Research in Pharmacy and Biosciences 4.8 (2017): 13-24.