Hepatic vasculature: a conceptual review.

The hepatic circulation is reviewed with emphasis on the role of hepatic blood vessels in hepatic and homeostatic functions. Contrasts are made with resistance, capacitance, and fluid exchange functions in other better known vascular beds. Hemodynamic changes that produce shifts in fluid exchange in other tissues are without effect in the liver. Elevations of hepatic venous pressure are transferred quantitatively to the sinusoids and result in prolonged, massive fluid filtration into the abdominal cavity. Other factors that are involved with control of fluid exchange are discussed. The liver contains a large volume of blood which can be rapidly mobilized during hemorrhage. The hepatic circulation is highly sensitive to changes in circulating blood volume and serves as a major buffer for expanded or contracted blood volume. Control of hepatic blood flow and the reciprocal relationship between portal and hepatic arterial flow is discussed. Changes in hepatic blood flow produce marked changes in hepatic clearance rates of a wide variety of compounds. It is concluded that the hepatic artery is not controlled by local tissue metabolism but rather is controlled by an, as yet unknown, mechanism that tends to maintain hepatic blood flow (and therefore clearance rates) constant.