Question 18

College Answer

The liver has a unique dual supply – a portal venous system, and a hepatic arterial system,
that floods the hepatic sinusoids and drain into the hepatic veins and then the inferior venacava. Knowledge of liver blood flow is essential for the understanding of certain 
pathophysiological mechanisms associated with disease and/or injury states of the liver. 
These two systems have unique characteristics, which most candidates seemed to have 
some knowledge of. A common area of weakness was the understanding/omission of the 
factors/mechanisms involved in the regulation of liver blood flow. 

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

Eipel, Christian, Kerstin Abshagen, and Brigitte Vollmar. "Regulation of hepatic blood flow: the hepatic arterial buffer response revisited." World journal of gastroenterology: WJG 16.48 (2010): 6046.

Abshagen, K., et al. "Anatomy and physiology of the hepatic circulation." PanVascular Medicine (2015): 3607-3629.

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.