Liver Anatomic Segmentation
The liver is divided into anatomic segments based on portal vein distribution (option d). 1 This segmentation system, known as the Couinaud classification, divides the liver into eight functionally independent segments, each with its own vascular inflow, biliary drainage, and venous outflow.
Portal Vein Distribution as the Basis for Segmentation
The portal vein distribution forms the primary basis for liver segmentation for several key reasons:
- Embryological development: Portal vein branching appears first during liver development, with arteriobiliary branching secondarily following the portal vein distribution 2
- Surgical relevance: Each segment receives its own branch of the portal vein, allowing for anatomical resections that preserve vascular integrity to remaining liver tissue
- Functional independence: The segments defined by portal vein distribution represent functionally independent units of liver tissue
Comparison with Other Vascular Structures
While other vascular structures are important in liver anatomy, they do not serve as the primary basis for segmentation:
- Hepatic arterial supply (option b): The hepatic artery follows portal vein distribution but is secondary in defining segments
- Hepatic venous drainage (option c): Hepatic veins run between segments rather than within them, forming the boundaries between segments rather than defining them
- Bile duct drainage (option a): Bile ducts follow the portal vein distribution but are not the primary basis for segmentation
Clinical Implications of Portal Vein-Based Segmentation
Portal vein-based segmentation has significant implications for liver surgery:
- Anatomic resections: Understanding portal territories allows surgeons to perform anatomic resections with preservation of vascular structures 3
- Surgical planning: Preoperative imaging to identify portal vein distribution is essential for planning liver resections 1
- Transplantation: Portal segmentation facilitates procedures such as partial and split-liver transplantation 1
Common Pitfalls and Considerations
- Anatomical variations: There are significant variations in segmental volumes (up to 24% from classically described volumes) 1
- Beyond the 8-segment concept: Recent research suggests the human liver contains an average of 20 (not 8) second-order portal venous territories 4, which explains why clinical observations sometimes don't match Couinaud's eight-segment concept
- Imaging interpretation: When surgical or imaging observations don't match Couinaud's scheme, clinicians should consider the inherent complexity of portal branching rather than automatically assuming anatomical variation 4
Understanding the portal vein distribution as the basis for liver segmentation is fundamental for safe hepatic surgery, accurate radiological interpretation, and optimal management of liver diseases.