Outline the anatomy and physiology of liver blood flow (60% of marks).
Explain the changes to drug metabolism when liver blood flow decreases (40% of marks).
A statement regarding the quantum of hepatic blood flow with recognition of the contributions
made by the Hepatic Artery and Portal Vein, drainage into the sinusoids before entering the
hepatic vein which drains into the IVC would have been a good start. Discussion was then
expected to revolve around how the liver blood flow is controlled. Answering this with respect
to intrinsic and extrinsic factors along with an understanding of the semi-reciprocal relationship
between hepatic arterial and portal venous blood flow would have rounded out a good answer
to the first part of the question.
The second part of the question required recognition that hepatic clearance is the product of
hepatic blood flow and the hepatic extraction ratio and considering the impact on drugs with a
high or a low extraction ratio.
Many answers failed to adequately mention how hepatic blood flow was controlled/regulated
thus limiting the marks available. Similarly, in the second part of the question, many candidates
spent considerable time mentioning the principals of drug metabolism rather than focusing on
the question asked.
The concept of hepatic drug clearance as the product of blood flow and its extraction ratio was
The anatomy and physiology of hepatic blood flow is a difficult topic to write a short coherent answer for. What follows is a summary of a summary, made as short as possible without losing what the author considers to be key elements. Even with this level of distillation, it ended up being humongous and unwieldy, such that no normal person could ever reproduce it in ten minutes, even if it were memorised and regurgitated verbatim. It is therefore remarkable that 55% of candidates passed this SAQ, and it would be very interesting to see what they wrote and which corners they cut.
As for the changes to drug metabolism when liver blood flow decreases:
where everything beyond the "×" symbol represents the hepatic extraction ratio
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Greenway, C. V., and R. D. Stark. "Hepatic vascular bed." Physiological reviews 51.1 (1971): 23-65.
Dancygier, Henryk. "Hepatic Circulation." Clinical Hepatology. Springer, Berlin, Heidelberg, 2010. 55-
Wilkinson, Grant R., and David G. Shand. "A physiological approach to hepatic drug clearance." Clinical Pharmacology & Therapeutics 18.4 (1975): 377-390.