Hepatic blood flow: morphologic aspects and physiologic regulation.

The study of the morphology of the hepatic circulation has given evidence that the liver consists of a large vascular delta formed by the confluence of the portal and arterial streams. Their arms, which subdivide the delta into lobar areas, start to run parallel and close to each other when they are still visible to the naked eye. Dwindled down to microscopic size, they become the scaffold of the parenchymal cell masses nestling between the microvessels. The arterioles, as they merge with the sinusoidal and portal channels, assume the role of organizing the microcirculation into units. These units are the vascular core of the structural and functional liver acini. It has now been demonstrated beyond doubt that a PO2 gradient exists in the hepatic vessels and tissues, decreasing from the site of the arteriolar rivulets joining the venous stream toward the site of their common egress via the terminal hepatic venules. The gradient permits the subdivision of the microscopic vascular units into three microcirculatory zones, each of them creating an appropriate microenvironment for specific enzymic and metabolic activity. The microcirculatory shifts in arterial flow from tide to ebb will cause change in the activity of the zones. These are essentially dynamic subdivisions of the metabolic activity in the large liver swamp. Here also start the tiny rivulets forming a green river, the bile stream, that runs in the opposite direction to the portal and hepatic arterial flow. It is to be expected that the quantity and quality of bile carrying important products back to the gastrointestinal area for digestion and absorption of fat are influenced by the tides in portal and arterial flow. All in all, it is evident that vascular morphology is the visual aspect of the dynamic blood flow, thus permitting us to perceive its functional orderliness, and to study the circulatory physiology in the hepatic delta. Means of measurement of hepatic blood flow have been reviewed and its methodological problems have been discussed. It was found that the term "estimated" hepatic blood flow is still justified. Also the relationship between hepatic blood flow and metabolism is not yet clear-cut. The role of the arterial and portal components of the hepatic circulation has been analyzed. There is a reciprocal relationship between arterial and portal volume flow; it is effectuated by the state of constriction or dilation of the mesenteric and hepatic arterioles, both under myogenic control. Portal blood delivers directly to the hepatocyte all water-soluble substances absorbed from the intestines or produced in the intestinal walls. The hepatic artery maintains an appropriate PO2 gradient between the acinar zones and flow of blood against increased tissue resistance; it assures a steady clearance of blood-borne substances, e.g., hormones and endogenous products. Regulation of arterial flow is less neural than neurohumoral; metabolites and bile salts exert additional effects on blood flow...