Anatomical considerations

High-pressure high-resistance hepatic artery blood flow mixes with low-pressure low-resistance portal vein blood to form sinusoid capillaries. These vessels are characterized by a thin fenestrated endothelium that allows easy transfer of blood constituents through the space of Disse and communication with hepatocyte and Kupffer cells. Within the liver, the triangular portal space is formed by ramifications of the portal vein, the hepatic artery, and the bile ducts. Between the portal spaces, centrilobular veins progressively mix to form the hepatic veins.

Several histological descriptions of this arrangement have been proposed:

1. the hepatic lobule, with a centripetal circulation towards the centrilobular vein, and a centrifugal circulation of the bile ducts;

2. the portal lobule, or portion of the hepatic parenchyma perfused by a portal space (with a centrifugal circulation and a centripetal biliary excretion);

3. the hepatic acinus.

The acinus represents a portion of the hepatic parenchyma between two centrilobular veins and is the functional unit of the liver, centered by a terminal portal venule, a terminal arteriole, a bile duct, lymphatic vessels, and nerves. Blood flows through the capillary sinusoids and is drained at the periphery of the acinus by terminal hepatic venules. The sinusoids are perfused by a mixture of hepatic arterial and portal venous blood joining at capillary level. Three different zones can be distinguished according to the perfusion of the liver: a well-perfused central zone close to the smaller vascular ramifications of the portal space, an intermediate zone, and a less perfused area close to the centrilobular veins.

Because of the dual vascularization within the liver originating from hepatic artery and portal vein, shunts and sphincters have been described. Vascular shunts exist between the hepatic artery, the portal vein or sinusoids, and the hepatic veins. The existence of sphincters at hepatic arteriolar and venous presinusoidal levels has also been suggested (Richardson..1982). Although specific muscular formation has not been formally identified, it has been suggested that arterial and venous presinusoidal 'sphincters' contract independently and regularly. This may be due to a myogenic phenomenon. Therefore a given sinusoid capillary can be vascularized by blood with different oxygen contents. Considering global flows, the portal venous flow Qpv, the outflow of the mesenteric circulation, is roughly 25 to 33 per cent of cardiac output, with Qpv representing two-thirds of total hepatic blood flow Qhep and hepatic arterial flow Qha representing the other third. However, because of the differences in arterial and venous oxygen contents, two-thirds of the hepatic oxygen supply depends on the hepatic arterial circulation.

Monitoring hepatic blood flow in humans is difficult; this is in part due to its dual circulation. Doppler techniques are promising, but are limited to patients undergoing surgery. In contrast, monitoring of hepatic venous oxygen saturation is relatively easily performed using a fiber-optic pulmonary artery catheter inserted into the hepatic vein, and is useful clinically because of the linear relationship between hepatic oxygen delivery and hepatic vein PO2. In the intensive care unit, this monitoring can help diagnose hepatic hypoperfusion and assess the response to therapies aimed at increasing liver blood flow.

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