Eight segments, each with its own portal inflow and biliary drainage
Mnemonic
Eight segments, numbered clockwise as seen from the front, starting with the caudate lobe as segment I.
- I - caudate
- II, III - left lateral, above and below
- IV - left medial (quadrate), often split into IVa and IVb
- V, VI, VII, VIII - right lobe, numbered anticlockwise from the front on the patient’s right
Each segment has its own portal pedicle (portal vein, hepatic artery and bile duct branch) and its own venous drainage, which is what makes segmental resection possible.
The true left and right lobes are divided by Cantlie’s line, running from the gallbladder fossa to the inferior vena cava, not by the falciform ligament, which is a surface feature only.
The caudate lobe is unique: it receives blood from both sides and drains directly into the inferior vena cava, which is why it hypertrophies in Budd-Chiari syndrome.
Expansion
The liver is divided into eight functionally independent segments, each supplied by a branch of the portal vein, hepatic artery and bile duct, and drained by a hepatic vein.
- Segment I - caudate lobe
- Segments II and III - left lateral section
- Segment IV - left medial section (quadrate lobe is IVb)
- Segments V to VIII - right lobe
The divisions are set by the three hepatic veins vertically and by the portal vein horizontally. The functional right-left division is Cantlie’s line, running from the gallbladder fossa to the inferior vena cava, not the falciform ligament, which is the anatomical division.
Segment I (caudate) is special: it receives blood from both sides and drains directly into the inferior vena cava through small veins. That is why it hypertrophies and is spared in Budd-Chiari syndrome, when the main hepatic veins are occluded.