Liver — Lobes, Couinaud Segments & Relations

The liver is the largest gland and largest solid viscus of the body (~1.5 kg in the adult). For NEET PG, the single most tested concept is the contrast between the traditional morphological lobes (defined by surface ligaments) and the functional left–right division along Cantlie's line (the plane of the middle hepatic vein). Add to this Couinaud's eight segments, the porta hepatis structures with their order, the Pringle manoeuvre, and liver trauma grading, and you have covered nearly every liver anatomy MCQ.

Surface anatomy, surfaces & ligaments

The liver occupies the right hypochondrium, epigastrium and part of the left hypochondrium, lying mostly under the cover of the lower ribs. It has two surfaces:

• Diaphragmatic surface — convex, related to the diaphragm; covered by peritoneum except along the bare area (between the two layers of the coronary ligament, bounded by the upper and lower layers and the right/left triangular ligaments). The bare area lies in direct contact with the diaphragm and is the site where the IVC is grooved.
• Visceral (inferior) surface — concave, faces down, back and to the left; bears the H-shaped arrangement of fissures and the porta hepatis.

The H-shaped grooves on the visceral surface are a favourite identification question:

Limb of the "H"	Structure occupying it
Right vertical limb (anterior)	Fossa for gallbladder
Right vertical limb (posterior)	Groove for IVC
Left vertical limb (anterior)	Fissure for ligamentum teres (obliterated left umbilical vein)
Left vertical limb (posterior)	Fissure for ligamentum venosum (obliterated ductus venosus)
Horizontal bar of the "H"	Porta hepatis

High-yield: Ligamentum teres = remnant of the left umbilical vein (carried oxygenated blood in fetus). Ligamentum venosum = remnant of the ductus venosus. Don't confuse the two.

Peritoneal attachments

• Falciform ligament — sickle-shaped fold connecting liver to anterior abdominal wall/diaphragm; its free lower margin carries the ligamentum teres. It superficially appears to divide the liver into right and left anatomical lobes — but this is not the functional plane.
• Coronary ligament — reflections enclosing the bare area; ends on each side as the right and left triangular ligaments.
• Lesser omentum — from the porta hepatis and fissure for ligamentum venosum to the lesser curvature of stomach; its free right margin contains the portal triad and forms the anterior boundary of the epiploic foramen (of Winslow).

Anatomical lobes vs functional lobes — Cantlie's line

This is the conceptual heart of the topic.

Morphological (anatomical) lobes are defined by surface peritoneal attachments:

• Right lobe (largest), left lobe, plus two accessory lobes seen on the visceral surface — the caudate lobe (posterosuperior, between IVC groove and fissure for ligamentum venosum) and the quadrate lobe (anteroinferior, between gallbladder fossa and fissure for ligamentum teres). By this scheme caudate and quadrate are part of the right anatomical lobe.

Functional (physiological/surgical) lobes are based on the blood supply and biliary drainage. The true plane separating the functional right and left livers is Cantlie's line — a plane running from the gallbladder fossa (notch) anteriorly to the groove for the IVC posteriorly. This plane contains the middle hepatic vein.

High-yield: Cantlie's line (the principal/portal plane) passes through the gallbladder fossa to the IVC and corresponds to the course of the middle hepatic vein. It separates the functional right and left lobes — NOT the falciform ligament.

By the functional scheme:

• The quadrate lobe functionally belongs to the left lobe (it is segment IVb, supplied by the left branches).
• The caudate lobe (Segment I) is unique — it has an independent / dual blood supply from both right and left portal/arterial branches and drains directly into the IVC by its own small hepatic veins, bypassing the three major hepatic veins.

Feature	Anatomical lobes	Functional lobes
Dividing plane	Falciform ligament + fissures	Cantlie's line (gallbladder → IVC)
Marker vein	None	Middle hepatic vein
Quadrate lobe belongs to	Right lobe	Left lobe (Seg IVb)
Caudate lobe	Right lobe	Autonomous (Seg I)
Basis	Surface peritoneal folds	Vascular & biliary territories

Couinaud's segments

The French surgeon Claude Couinaud divided the liver into 8 functionally independent segments, each with its own portal pedicle (branch of portal vein + hepatic artery + bile duct) and a hepatic venous drainage. Because each segment is self-sufficient, a surgeon can resect a single segment (segmentectomy) without devascularising the rest — the conceptual basis of modern hepatic resection.

The three hepatic veins divide the liver into four vertical sectors; the portal vein plane (transverse) divides these into superior and inferior segments → 8 segments.

The organising scheme:

• Right hepatic vein → separates the right lobe into anterior (V, VIII) and posterior (VI, VII) sectors.
• Middle hepatic vein (in Cantlie's plane) → separates right from left liver.
• Left hepatic vein → separates left lobe into medial (IV) and lateral (II, III) sectors.
• The portal venous plane (porta hepatis level) divides each sector into superior (above) and inferior (below) segments.

Segment	Name / location	Notes
I	Caudate lobe	Independent supply; drains directly to IVC
II	Left lateral superior
III	Left lateral inferior
IVa	Left medial superior	Part of quadrate region
IVb	Left medial inferior	= quadrate lobe functionally
V	Right anterior inferior	Adjacent to gallbladder fossa
VI	Right posterior inferior
VII	Right posterior superior
VIII	Right anterior superior	Most superior right segment

Numbering flow: Start at caudate (I), then move anticlockwise (when viewed in the standard anterior CT orientation) — II → III → IV in the left lobe, then V → VI → VII → VIII in the right lobe.

High-yield: Segment I (caudate lobe) is the one most often spared in Budd–Chiari syndrome because its venous blood drains directly into the IVC via separate veins and not through the occluded major hepatic veins — leading to caudate lobe hypertrophy.

High-yield: Each Couinaud segment has its own portal pedicle and independent vascular inflow, which is precisely why segment-oriented resection is possible. This single sentence answers many "why" MCQs.

Mnemonic for remembering the right-lobe superior/inferior split: "58 above the line, 67 below" is wrong — fix it as superior = VII, VIII (and II, IVa) and inferior = V, VI (and III, IVb). A cleaner memory hook: on an axial CT at the level of the hepatic veins/IVC, you see the superior segments (II, IVa, VII, VIII); at the level just below the portal bifurcation you see the inferior segments (III, IVb, V, VI).

Blood supply

The liver enjoys a dual blood supply:

• Hepatic artery proper — ~25–30% of inflow but ~50% of oxygen; a branch of the common hepatic artery from the coeliac trunk.
• Portal vein — ~70–75% of inflow (deoxygenated, nutrient-rich), formed behind the neck of the pancreas by union of the superior mesenteric vein and splenic vein.

The hepatic artery, portal vein and bile duct enter at the porta hepatis and branch in parallel (the portal triad) down to the level of the segment, sheathed by the perivascular fibrous capsule of Glisson.

High-yield: Portal vein supplies the majority of blood volume (~75%), but the hepatic artery supplies about half the oxygen. Both supply roughly equal oxygen in the fed/normal state.

Venous drainage

Three major hepatic veins — right, middle, left — drain into the IVC just below the diaphragm. The left and middle hepatic veins commonly form a common trunk before entering the IVC. In addition, several small (accessory) hepatic veins drain the caudate lobe directly into the IVC.

Porta hepatis — structures and their order

The porta hepatis is the transverse fissure on the visceral surface (the bar of the "H"). It transmits, from before backwards / and in the free margin of the lesser omentum:

Relations in the free edge of the lesser omentum (anterior to posterior):

1. Bile duct — anterior and to the right
2. Hepatic artery — anterior and to the left
3. Portal vein — posterior (behind both)

High-yield (classic MCQ): In the free margin of the lesser omentum / at the porta hepatis: Portal vein is posterior, hepatic artery is to the left (anterior), bile duct is to the right (anterior). Mnemonic: "D-A-V" front to back, or "Duck (Duct) to the right, Artery to the left, Vein behind."

Posterior to the portal triad lies the epiploic foramen (of Winslow), whose boundaries are:

Boundary	Structure
Anterior	Free margin of lesser omentum (portal triad)
Posterior	IVC
Superior	Caudate lobe of liver
Inferior	First part of duodenum / hepatic artery

Pringle manoeuvre

The Pringle manoeuvre is the surgical compression/clamping of the hepatoduodenal ligament (free margin of the lesser omentum) between finger and thumb (or with an atraumatic vascular clamp) placed through the epiploic foramen of Winslow. It occludes the portal vein and hepatic artery simultaneously, controlling inflow bleeding during liver surgery or trauma.

Approach flow: Identify bleeding liver → pass finger through foramen of Winslow → compress hepatoduodenal ligament (occludes hepatic artery + portal vein) → bleeding from cut surface continues → suspect injury to hepatic veins / retrohepatic IVC (which are NOT controlled by Pringle).

High-yield: If bleeding persists despite a Pringle manoeuvre, the source is the hepatic veins or retrohepatic IVC (the outflow, not the inflow), because Pringle controls only the inflow (portal vein + hepatic artery). This is one of the most repeated surgical-anatomy MCQs.

High-yield: Pringle compresses the hepatoduodenal ligament containing the portal triad; the clamp is introduced via the epiploic foramen of Winslow whose posterior boundary is the IVC.

Liver trauma grading (AAST)

Liver injury is graded by the American Association for the Surgery of Trauma (AAST) Organ Injury Scale, I–VI:

Grade	Key feature
I	Subcapsular haematoma <10% surface area; capsular tear <1 cm deep
II	Subcapsular haematoma 10–50%; laceration 1–3 cm deep, <10 cm length
III	Subcapsular haematoma >50% or ruptured; laceration >3 cm deep
IV	Parenchymal disruption involving 25–75% of a lobe (or 1–3 Couinaud segments)
V	Parenchymal disruption >75% of a lobe (or >3 segments); juxtahepatic venous injury (retrohepatic IVC / central major hepatic veins)
VI	Hepatic avulsion (incompatible with survival)

Management is overwhelmingly non-operative in the haemodynamically stable patient (the liver is the most commonly injured solid organ in penetrating abdominal trauma; the spleen edges it in blunt trauma in many series, though the liver is increasingly reported as most commonly injured overall). Indication for operative management is haemodynamic instability not responding to resuscitation, or peritonitis.

High-yield: Grade V liver injury involves juxtahepatic (retrohepatic) venous injury — exactly the bleeding a Pringle manoeuvre cannot control; such injuries may need atriocaval (Schrock) shunt or perihepatic packing and damage-control surgery.

Important relations (visceral impressions)

On the visceral surface of the right and left lobes you can identify the following impressions — occasionally asked:

Impression	Made by
Gastric impression	Anterior surface of stomach (left lobe)
Oesophageal groove	Abdominal oesophagus
Duodenal impression	First part of duodenum
Colic impression	Right colic flexure
Renal impression	Right kidney
Suprarenal impression	Right suprarenal gland (bare area)
Pyloric / lesser curvature	Pylorus, lesser omentum

Development & clinical correlates (quick)

• The liver arises from the hepatic diverticulum (foregut endoderm) growing into the septum transversum; the mesoderm of septum transversum forms connective tissue, Kupffer cells and haematopoietic tissue.
• Cantlie's line correlates radiologically with the middle hepatic vein on CT/MRI — used to plan right vs left hepatectomy.
• Riedel's lobe — a tongue-like inferior projection of the right lobe, a normal variant mimicking a mass/hepatomegaly, commoner in women.

Recently asked / exam angle

• "Cantlie's line passes through?" → gallbladder fossa to IVC, along the middle hepatic vein (separates functional R and L lobes).
• "Quadrate lobe functionally belongs to?" → Left lobe (Segment IVb).
• "Caudate lobe is which Couinaud segment / why spared in Budd–Chiari?" → Segment I; drains directly into IVC.
• "Order of structures at porta hepatis / free edge of lesser omentum?" → Portal vein posterior, hepatic artery left, bile duct right.
• "Pringle manoeuvre controls / fails to control?" → controls portal vein + hepatic artery; fails for hepatic vein / IVC bleeding.
• "Boundaries of epiploic foramen of Winslow — posterior?" → IVC.
• "Liver receives most of its blood / oxygen from?" → most blood from portal vein (~75%); ~half the oxygen from hepatic artery.
• "Number of Couinaud segments?" → 8.
• "AAST Grade V liver injury?" → juxtahepatic venous disruption / >75% of a lobe.
• "Ligamentum teres is a remnant of?" → left umbilical vein; ligamentum venosum = ductus venosus.

Rapid revision

1. Liver = largest gland; dual blood supply — portal vein (~75% blood), hepatic artery (~50% oxygen).
2. Cantlie's line = gallbladder fossa → IVC = plane of the middle hepatic vein = true functional R/L division (NOT falciform ligament).
3. Couinaud = 8 segments, each with an independent portal pedicle → allows segmentectomy.
4. Caudate lobe = Segment I, drains directly into IVC, spared/hypertrophies in Budd–Chiari.
5. Quadrate lobe = Segment IVb, functionally part of the left lobe.
6. Three hepatic veins (R, M, L) drain into IVC; left + middle often share a common trunk.
7. Porta hepatis order: bile Duct right, hepatic Artery left, portal Vein posterior ("DAV").
8. Epiploic foramen of Winslow: anterior = portal triad, posterior = IVC, superior = caudate lobe, inferior = D1/hepatic artery.
9. Pringle manoeuvre = clamp hepatoduodenal ligament (via foramen of Winslow) → occludes PV + hepatic artery; persistent bleed → hepatic vein/IVC injury.
10. Ligamentum teres = left umbilical vein; ligamentum venosum = ductus venosus.
11. AAST Grade V liver trauma = retrohepatic/juxtahepatic venous injury (Pringle won't help → packing / Schrock shunt).
12. Riedel's lobe = benign inferior tongue of the right lobe; bare area lies between coronary ligament layers, contacting the diaphragm and grooving the IVC.