The liver can be viewed from two different perspectives - anatomical and functional.

Anatomically, the liver is divided into four lobes based on its visual appearance.
Functionally, it has two lobes based on the distribution of portal and hepatic veins.

Functionally, the liver can be further divided into four smaller parts called sectors. Each sector is usually subdivided into even smaller pieces known as segments.

The intersectoral planes between these sectors are termed portal fissures. They are not visible externally on the liver surfaces and are only identifiable in the parenchyma of the liver. The three principal hepatic veins run through these fissures.

Note: The current understanding of the functional anatomy of the liver encompasses two models - a French model and an American one. In the French model, the liver is divided based on the distribution of portal and hepatic veins, while the American model divides it according to the branching patterns of bile ducts. In the following slides, we review the French model and the Couinaud classification, as it is the most widely used system for the functional division of the liver nowadays. In 1957, a French surgeon named Claude Couinaud described a classification of eight independent functional units (segments) of the liver according to the branching pattern of the portal vein and hepatic veins within the hepatic parenchyma. All segments are provided by a vascular inflow, outflow and biliary drainage, as each contains a branch of the portal vein, hepatic artery and biliary duct. The outflow happens via the hepatic veins found in the periphery of the segments.

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Anatomical and functional divisions of liver