## Distinction Between Functional and Anatomical Left Lobes ### Anatomical vs. Functional Division The liver is divided into lobes by two different classification systems that do not correspond anatomically. **Key Point:** The anatomical left lobe is defined by the falciform ligament and includes only segments II and III. The functional (surgical) left lobe is defined by the plane of the middle hepatic vein and includes segments II, III, and IV (quadrate lobe). ### Why Size Matters Clinically The functional left lobe is significantly larger than the anatomical left lobe because it includes segment IV (the quadrate lobe), which lies to the right of the falciform ligament but is supplied by the left portal vein and left hepatic artery. This functional division is clinically relevant for: - Hepatic resection planning - Living donor liver transplantation - Assessment of hepatic blood supply ### Segmental Anatomy | Division | Segments Included | Boundary | Clinical Use | |----------|------------------|----------|---------------| | **Anatomical Left Lobe** | II, III | Falciform ligament | Gross anatomy | | **Functional Left Lobe** | II, III, IV | Middle hepatic vein plane | Surgical planning | | **Anatomical Right Lobe** | V, VI, VII, VIII, I | Falciform ligament | Gross anatomy | | **Functional Right Lobe** | V, VI, VII, VIII | Middle hepatic vein plane | Surgical planning | **High-Yield:** In hepatic surgery, the functional division is preferred because it respects vascular and biliary anatomy. The functional left lobe extends across the midline to include segment IV (quadrate lobe), making it larger than the anatomical left lobe. **Clinical Pearl:** During hepatectomy, surgeons use the plane of the middle hepatic vein (Cantlie's line) to separate functional lobes, not the falciform ligament, because this preserves vascular supply to the remaining liver. **Mnemonic:** **FLIV** = Functional Left Includes IV (quadrate lobe) 
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