Hepatic blood supply

The liver is unique in that it has dual blood supply from both the portal vein and the hepatic artery; in total the liver receives 25% of cardiac output. The portal vein is derived from the confluence of the superior mesenteric vein, inferior mesenteric vein, and splenic vein. The hepatic artery is a branch from the celiac trunk off of the descending aorta.

Under normal conditions:

– Portal vein = 70% total hepatic blood flow, 50% hepatic O₂ supply

– Hepatic artery = 30% total hepatic blood flow, 50% hepatic O₂ supply

Portal vein flow depends directly on systemic blood pressure and cardiac output. If portal blood flow is reduced by 50% or greater, the hepatic artery dilates. This compensatory response is called the hepatic arterial buffer response and is mediated by adenosine; it is stimulated by acidosis, hypoxemia, and hypercarbia.

Hepatic blood flow is determined DIRECTLY by hepatic perfusion pressure (HPP) and INDIRECTLY by splanchnic vascular resistance (innervated by the sympathetic nervous system).

↑hepatic blood flow = ↑HPP or ↓splanchnic vascular resistance

↓hepatic blood flow = ↓HPP or ↑splanchnic vascular resistance

Hepatic perfusion pressure is DIRECTLY related to mean arterial pressure (MAP) and portal vein pressure, and INDIRECTLY related to hepatic vein pressure. Importantly, hepatic vein pressure is increased with positive pressure ventilation, CHF (particularly right heart failure), and volume overload (anything that increases CVP). Splanchnic vascular resistance is increased – and therefore hepatic blood flow is decreased – with: pain, arterial hypoxemia, and surgical stress.