The Couinaud classification system divides the liver into eight, functionally independent segments each with a center consisting of portal vein and hepatic artery branches as well as bile duct drainage. The periphery of each segment is where vascular outflow occurs via the hepatic veins. The numbering system starts with the caudate lobe (segment I, on the posterior of the liver and not seen in this image) and carried out in a clockwise fashion. Of note, segments VI and VII were drawn for the purpose of illustration. They will typically not be seen on an anterior view of the liver since they are posterior structures.
The right hepatic vein divides the right lobe into anterior (V and VIII) and posterior segments (VI and VII). The left hepatic vein divides the left lobe into lateral (II and III) and medial (IVa and IVb) segments. It follows that the middle hepatic vein splits the liver into right and left lobes. The portal vein divides the liver into superior and inferior segments.
Of note, the gallbladder (GB) separates segments V and IVb and the falciform ligament divides the left lobe into medial and lateral segments (just like the left hepatic vein).
Knowing the liver segments is helpful to localize pathology and to discuss planes for surgical resection (ie, extended right hepatectomy, left medial sectionectomy, etc.)
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