How many liver segments are there, and how are they categorized?

The liver is anatomically divided into eight segments according to the Couinaud classification, which categorizes the segments based on the distribution of the hepatic blood supply and bile drainage. This classification recognizes the liver's dual blood supply from the hepatic artery and the portal vein, allowing each segment to function somewhat independently from the others in terms of vascularization.

In the Couinaud system, the segments are divided as follows: the first two segments correspond to the caudate lobe (segments I - caudate), while segments II through VIII comprise the left and right lobes. Specifically, segments II and III are in the left lobe, and segments IVa, IVb, V, VI, VII, and VIII correspond to the right lobe. This categorization is clinically significant for surgical procedures, including resections and transplantations, as it allows surgeons to reference specific liver areas with respect to their vascular supply and biliary drainage.

Understanding this segmentation is crucial for diagnosing liver disease, planning surgeries, and performing liver interventions effectively. The detailed division into eight segments provides a clear and functional way to think about the liver's anatomy and operations within it, including the metabolism and production of bile.