Question 4

Briefly outline the functions of the liver.

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College Answer

This question was generally well answered with a good response being in some structured format, e.g. a mention, followed by a description for each function of the liver. For questions asking to outline a particular topic, a general overview of the topic is expected and not merely a “dot-point” list of the functions of the liver without actually delving into the way the liver does those functions. In general candidates should avoid making broad-brush statements, which do not get them any marks like “the Liver is the major organ in the body”. Candidates were expected to list, and provide an overview for each, function of the liver.

Discussion

The candidates were only imitating published authors, who cannot seem to resist an opening line like "the liver is the largest visceral organ in mammals", even as they submit their articles for review by competent specialist peers who have researched the liver for their entire professional life. And like these authors, the triteness of these statements had done nothing to hurt the candidates' chances, as almost 90% of them passed this question. Heartened, future trainees can continue to write "much liver large", "is gland", "so organ", without any risk of serious penalty. 

  • 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)

References

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.