What divides the liver into segments, the hepatic (hepatic) vein, portal (portal) vein, hepatic (hepatic) artery, or biliary divisions?

Liver Segmentation: The Portal Vein Divides the Liver into Segments

The liver is divided into segments by the portal vein (option B). The portal vein and its branches serve as the anatomical basis for the functional segmentation of the liver, as defined by the widely used Couinaud classification system 1.

Anatomical Basis of Liver Segmentation

The portal vein plays the primary role in liver segmentation for several key reasons:

• The portal vein branches divide the liver into functionally independent units, each with its own vascular inflow, biliary drainage, and lymphatic drainage 1
• Each segment receives its own branch of the portal vein, which defines the territory supplied by that branch 2
• In the Couinaud classification method, each third-order branch of the portal vein defines the supplied territory of a corresponding liver segment 2

Portal Vein Branching Pattern

The portal vein typically divides into right and left branches at the porta hepatis:

• The left branch supplies segments I-IV
• The right branch further divides into anterior and posterior branches supplying segments V-VIII 3

Why Other Options Are Incorrect

1. Hepatic Vein (option A): While hepatic veins are important landmarks that help define the boundaries between segments, they do not serve as the primary basis for segmentation. The hepatic veins run between segments (intersegmental) rather than within them 4. Studies have shown that there is no consistent spatial correlation between hepatic vein territories and Couinaud segments 4.

2. Hepatic Artery (option C): The hepatic artery follows the same branching pattern as the portal vein but is not the primary determinant of segmentation. It provides only about 25% of the blood supply to the liver and follows the portal vein branches 3.

3. Biliary Divisions (option D): The biliary tree also follows the portal vein branching pattern but is not used as the primary basis for liver segmentation. Bile ducts run alongside the portal vein branches within the segments 3.

Clinical Significance of Portal Vein-Based Segmentation

This portal vein-based segmentation has critical implications for:

1. Liver Surgery:

   • Allows for anatomical resections following the portal vein territories 3
   • Enables precise planning of segmentectomies and lobectomies 3

2. Portal Vein Embolization (PVE):

   • Used to induce hypertrophy of the future liver remnant before major hepatectomy 3
   • The procedure involves occluding portal venous flow to segments planned for resection 3

3. Liver Transplantation:

   • Split liver transplantation divides the liver based on portal vein territories 3
   • Living donor liver transplantation typically uses the right lobe (segments V-VIII) or left lateral segments (II-III) based on portal vein anatomy 3

Advanced Concepts and Variations

It's worth noting that recent research has challenged the traditional concept of exactly eight liver segments. Studies on portal vein casts have shown that the human liver actually contains an average of 20 second-order portal venous territories, not just 8 1. This "1-2-20 concept" explains many of the inconsistencies observed during surgery and imaging that don't fit perfectly with Couinaud's eight-segment model 1.

The portal vein-based segmentation remains the gold standard for understanding liver anatomy in both surgical planning and radiological assessment, despite these nuances in the exact number of segments.