What does the Couinaud classification system divide the liver into?

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The liver is divided into eight functionally independent segments in Couinaud classification, based on the branching patterns of the portal triad and hepatic veins. Each segment has its own inflow from a portal pedicle, its own venous outflow to a specific hepatic vein, and its own biliary drainage, enabling precise surgical planning and localization. The segmentation planes follow the distribution of the portal branches and hepatic veins and align with Cantlie's line to separate left and right components; the numbering starts with the caudate lobe as the first segment, then the left lobe segments II–IV (IVa and IVb subdivisions), and the right lobe segments V–VIII (with the anterior/posterior and superior/inferior subdivisions). This functional approach differs from external anatomy ideas like lobes or quadrants or from the hepatic acinar zones, which describe different aspects of liver organization.