College Answer

A correct description of the vascular anatomy; the contribution and composition of hepatic
artery and portal vein flow to total hepatic flow and how this is regulated would be awarded
with a pass. An answer that expanded on these main points received additional marks. The
interdependence of hepatic artery and portal vein flow was not appreciated by any candidate.
Either candidates knew the answer to this question or they did not. Some candidate(s) tried
to guess at what the anatomy might be. This attracted no marks. Many candidates lacked
sufficient knowledge to pass this question.

Discussion

• Hepatic blood supply:
  • From the hepatic artery (a branch of the coeliac trunk)
    • Under aortic pressure (MAP ~65-90 mmHg)
    • 30-40% of the blood flow (SvO₂= 85%; 40-50% of the DO₂)
  • From the portal vein (confluence of mesenteric and splenic veins)
    • Valveless, low pressure venous system (8-10 mmHg)
    • 70% of the total blood flow (SaO₂=95%;  50-60% of the DO₂)
  • Total hepatic blood flow: 1200-1500ml/min, or 100ml/100g/minute 
• Hepatic microcirculation:
  • Hepatic arterioles and portal venules join to form hepatic sinusoids
    • Low pressure, to prevent retrograde flow in the valveless portal system
    • Low flow velocity, to enhance extraction of oxygen and other molecules of interest
• Hepatic blood flow regulation
  • Portal venous flow regulation:
    • Flow rate is mainly determined by splanchnic arterial flow rate
    • Resistance changes in response to:
      • Humoural signals (eg. catecholamines), in shock
      • Local endocrine signals (eg. VIP and secretin), causing portal vasodilation following a meal
  • Hepatic arterial flow regulation:
    • Standard arterial regulatory mechanisms: myogenic, flow(shear)-mediated, conducted vasomotor responses, immunologically mediated by inflammatory molecules.
    • Hepatic arterial buffer response: hepatic arterial flow increases if portal venous flow decreases; mediated by adenosine
• External factors which influence hepatic blood flow:
  • Venous return: affects hepatic venous drainage (eg. during positive pressure ventilation or heart failure)
  • Cardiac output: influences hepatic arterial flow directly, and portal flow indirectly (eg. in heart failue)
  • Shock states and exercise: decrease splanchnic blood flow, both portal and hepatic