Liver Anatomy — LIMES™

The liver has two coexisting anatomical frameworks. The classical descriptive anatomy divides the liver into right and left lobes by the falciform ligament — a system inherited from gross anatomy and still used in everyday clinical language. This division is anatomically convenient but surgically misleading: the falciform does not correspond to any major vascular boundary.

The Couinaud functional anatomy, described by French surgeon Claude Couinaud in 1957, divides the liver into 8 independent segments based on actual vascular territory — each segment has its own portal vein branch (inflow), hepatic artery branch (inflow), bile duct (drainage), and hepatic vein tributary (outflow). This is the system surgeons, radiologists, and transplant hepatologists use. When an imaging report or operative note references a segment number, it is referring to Couinaud.

Key Principle

Because each Couinaud segment has independent inflow and outflow, individual segments or combinations of segments can be resected while preserving the remainder — this is the anatomical basis for partial hepatectomy, right hepatectomy, left lateral sectionectomy, and living donor liver transplantation (LDLT).

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How does Couinaud segmental anatomy differ from classical lobe-based anatomy, and why does it matter surgically?

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The eight Couinaud segments are numbered I through VIII in a roughly clockwise fashion when viewed from the anterior (frontal) perspective. Each is anatomically and functionally independent.

Couinaud liver segmentation diagram showing all eight segments (I–VIII) with colour-coded sections, hepatic vein planes, and lobe labels — anterior and posterior views

Couinaud classification — anterior and posterior views with segment numbers, lobe designations, and section boundaries. Image: BodyParts3D, © The Database Center for Life Science (DBCLS), licensed under CC BY-SA 2.1 JP.

Segment	Location	Key Notes
I — Caudate lobe	Posterior; sits between IVC and portal vein	Unique anatomy: drains directly into IVC via multiple small hepatic veins (not via the three major hepatic veins). Relatively preserved in Budd-Chiari syndrome due to independent outflow. Frequently involved in hilar cholangiocarcinoma (Klatskin tumor).
II	Left lateral lobe, superior	Together with segment III, forms the left lateral section — the standard graft for pediatric LDLT recipients (~15–20% of total liver volume).
III	Left lateral lobe, inferior	Together with segment II, forms the left lateral section. The lateral sectionectomy (II+III resection) is one of the most commonly performed anatomic hepatectomies.
IV (IVa = superior, IVb = inferior)	Left medial lobe; between falciform ligament and main scissura	IVb lies adjacent to the gallbladder fossa. Segment IV is included in the left lobe graft (II+III+IV) used in adult-to-adult LDLT when the right lobe is too large. Involved in central liver tumors near the bile duct confluence.
V	Right anterior lobe, inferior	Adjacent to the gallbladder. Right hepatectomy begins here. Drained by the middle hepatic vein (shared with VIII) and right hepatic vein.
VI	Right posterior lobe, inferior	Together with VII forms the right posterior section. Drained by right hepatic vein. Right posterior sectionectomy (VI+VII) is anatomically feasible for posteriorly located tumors.
VII	Right posterior lobe, superior	Sits adjacent to the right hepatic vein and IVC superiorly. Tumors here may involve the right hepatic vein, affecting resectability.
VIII	Right anterior lobe, superior	Adjacent to IVC and right/middle hepatic veins; critically located. Part of the right lobe LDLT graft (V+VI+VII+VIII), which comprises ~60–65% of total liver volume.

LDLT Donor Segments at a Glance

Right lobe graft (adult-to-adult): Segments V, VI, VII, VIII (~60–65% of liver volume)
Left lobe graft: Segments II, III, IV (~30–35%)
Left lateral section graft (pediatric): Segments II, III (~15–20%)

The right lobe is preferred for adult LDLT because it provides sufficient graft-to-recipient weight ratio (GRWR >0.8%). The donor retains the left lobe, which regenerates to near-normal volume within 6–8 weeks.

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A 32-year-old healthy volunteer is evaluated for adult-to-adult living donor liver transplant. Why is the right lobe (segments V–VIII) the standard graft rather than the left lobe (segments II–IV), and what key threshold must be met?

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Arterial Supply

Standard hepatic arterial anatomy is present in only about 55–60% of patients. The remainder have anatomical variants that are critically important in transplant surgery, hepatic resection, and embolization procedures.

Standard Anatomy

Celiac trunk → common hepatic artery → proper hepatic artery → right and left hepatic arteries, supplying the corresponding lobes. The gastroduodenal artery (GDA) arises from the common hepatic artery before the bifurcation and serves as an important anatomical landmark.

Replaced Right Hepatic Artery (from SMA)

rRHA — ~15–20%

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Replaced Left Hepatic Artery (from LGA)

rLHA — ~12–15%

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Accessory Variant Arteries

Supplementary — ~10%

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Pre-operative CT angiography (CTA) or MR angiography is standard practice before any major hepatectomy or living donor evaluation specifically to map arterial anatomy.

Portal Venous System

Portal Vein Formation and Flow

Superior mesenteric vein (SMV) + splenic vein → portal vein (formed posterior to the neck of the pancreas)
Portal vein enters the porta hepatis and bifurcates into right and left portal veins.
Portal vein provides approximately 75% of hepatic blood flow (high volume, lower oxygen content) — the hepatic artery provides the remaining 25% (lower volume, high oxygen).
Portal pressure measured as hepatic venous pressure gradient (HVPG) via hepatic vein wedge/free pressure measurement.

HVPG thresholds:

≥6 mmHg — portal hypertension (abnormal)
≥10 mmHg — clinically significant portal hypertension (CSPH); risk of complications escalates
≥12 mmHg — threshold associated with variceal formation and bleeding risk; also associated with increased risk of decompensation

Hepatic Veins

Three major hepatic veins drain directly into the inferior vena cava (IVC) at the level of the hepatic confluence, just below the right atrium. There is no portal pedicle structure associated with hepatic veins — they travel within the parenchyma between segments.

The Three Hepatic Veins

Right hepatic vein (RHV): Drains segments V, VI, VII, VIII. Largest of the three. Enters the IVC directly on the right. Must be controlled in right hepatectomy.
Middle hepatic vein (MHV): Drains segments IV, V, and VIII. Courses along the main hepatic scissura (principal plane). Often the key vein in right hepatectomy controversy — whether to include it in the right lobe graft affects outflow and left remnant venous drainage.
Left hepatic vein (LHV): Drains segments II and III. Frequently joins the middle hepatic vein to form a common trunk before entering the IVC (~80% of individuals).

Surgical Scissurae — Planes of Resection Defined by Hepatic Vein Courses

Main scissura (principal plane): Runs through the gallbladder fossa to the IVC; courses along the middle hepatic vein. Divides the right and left functional hemilivers. This is the plane of a right or left hepatectomy.
Right scissura: Separates right anterior (V/VIII) from right posterior (VI/VII) sections; courses along the right hepatic vein.
Left scissura: Corresponds to the falciform ligament; separates left lateral (II/III) from left medial (IV) sections; courses along the left hepatic vein.

These scissurae are the bloodless planes that surgeons follow during anatomic resections.

Portal Pedicle Concept

Within the liver parenchyma, the portal vein branch, hepatic artery branch, and bile duct travel together, sheathed in Glisson's capsule extensions — this triad is called the portal pedicle.

This anatomical grouping means that Couinaud's segmental anatomy applies equally to all three structures — each segment's portal pedicle can be approached and controlled as a unit during anatomic resection (the "Glissonian approach").

Biliary Drainage Pathway

Right hepatic duct: Formed by the anterior sector duct (segments V + VIII) and posterior sector duct (segments VI + VII)
Left hepatic duct: Drains segments II, III, and IV; longer extrahepatic course than the right (surgically advantageous — allows more length for anastomosis)
Right HD + Left HD → Common hepatic duct (CHD)
CHD + Cystic duct → Common bile duct (CBD)
CBD → Ampulla of Vater (enters second portion of duodenum, sharing the opening with the pancreatic duct via the sphincter of Oddi)
Calot's triangle: Bounded by the cystic duct (inferolaterally), common hepatic duct (medially), and inferior edge of the liver (superiorly) — the critical dissection zone in laparoscopic cholecystectomy; the cystic artery is identified and ligated here

Bile Duct Blood Supply — The Most Important Concept in Transplant Hepatology

The bile ducts receive their blood supply exclusively from arterial sources — primarily the hepatic arteries, via a fine peribiliary vascular plexus. There is no portal venous contribution to bile duct blood supply.

This explains why hepatic artery thrombosis (HAT) after liver transplantation causes biliary ischemia and necrosis (ischemic cholangiopathy) — even when the liver parenchyma itself initially appears viable (because parenchyma tolerates some portal perfusion alone). HAT post-LT presents as bile leaks, strictures, cast syndrome, and ultimately graft loss if untreated. It is the most feared early vascular complication of liver transplantation.

Biliary Anatomical Variants — High-Yield for LDLT and Cholecystectomy

Aberrant right posterior sectoral duct (~10–15%): The duct draining segments VI+VII inserts anomalously into the common hepatic duct (or even the cystic duct) rather than joining the anterior sectoral duct to form a proper right hepatic duct. This variant is a major risk factor for bile duct injury at cholecystectomy and creates reconstruction challenges in right lobe LDLT — the donor may have two separate bile duct openings requiring two anastomoses.
Low cystic duct insertion: The cystic duct joins the CBD very near the ampulla rather than at the mid-CBD. This is a significant risk factor for bile duct injury during cholecystectomy if the anatomy is not carefully delineated — the low-inserting cystic duct can be mistaken for the CBD.
Pre-operative MRCP (MR cholangiopancreatography) is standard in LDLT donor evaluation to characterize biliary anatomy.

The liver is held in position by peritoneal reflections and ligamentous attachments. These ligaments are relevant landmarks in abdominal surgery, laparoscopic cholecystectomy, and hepatic mobilization.

Ligament	Contents	Clinical Note
Falciform ligament	Ligamentum teres (obliterated left umbilical vein) within its free edge	Divides the right from left anatomical lobe on the anterior surface. In portal hypertension, the ligamentum teres may recanalize, producing paraumbilical varices (caput medusae). Also useful as a landmark for the left scissura. Divided during left hepatic mobilization.
Gastrohepatic ligament (lesser omentum, hepatogastric portion)	Usually contains only fat and small vessels; may contain a replaced left hepatic artery from the left gastric artery (~12–15%)	Must be inspected and palpated before dividing in any left hepatectomy or gastric surgery. Blindly dividing this ligament in a patient with a replaced left hepatic artery will devascularize segments II, III, and IV.
Hepatoduodenal ligament (lesser omentum, hepatoduodenal portion)	Portal vein (posterior), hepatic artery (anteromedial), and common bile duct (anterolateral) — forms the free edge of the lesser omentum (right border of the epiploic foramen of Winslow)	Pringle maneuver: Manual or clamp compression of the hepatoduodenal ligament at the epiploic foramen occludes hepatic inflow (both portal vein and hepatic artery simultaneously), reducing blood loss during hepatic parenchymal transection. The liver can typically tolerate 15–20 minutes of warm ischemia under Pringle; intermittent clamping extends this.
Coronary ligaments and triangular ligaments	Peritoneal reflections; bare area of liver (where posterior surface lacks peritoneal covering)	Divided during hepatic mobilization (right or left). Division of the right triangular ligament is required for right hepatic mobilization in right hepatectomy and major right-sided resections. The bare area of the liver is where the liver is directly in contact with the diaphragm.

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What is the Pringle maneuver and why does it reduce intraoperative blood loss?

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