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Liver Segments
The liver is divided functionally into eight Couinaud segments based on the distribution of portal venous, hepatic arterial and biliary branches. Each segment has relatively independent vascular inflow and biliary drainage, while the major hepatic veins generally occupy intersegmental planes.
RegionAbdomen
SystemDigestive System
The liver is divided internally into functionally distinct regions called hepatic segments. The most widely used system is the Couinaud classification, which divides the liver into eight functionally independent segments numbered I through VIII.
This segmentation is based primarily on the branching pattern of the portal vein and its accompanying hepatic arterial and biliary branches. Each segment receives a relatively independent portal pedicle containing branches of the portal vein, hepatic artery, and bile duct.
The major hepatic veins generally travel between these portal territories and therefore define important intersegmental planes. This arrangement allows portions of the liver to be surgically removed while preserving vascular inflow and biliary drainage to the remaining segments.
Couinaud Classification
The Couinaud system divides the liver into eight numbered segments:
- Segment I: Caudate segment
- Segment II: Left lateral superior segment
- Segment III: Left lateral inferior segment
- Segment IV: Left medial segment
- Segment V: Right anterior inferior segment
- Segment VI: Right posterior inferior segment
- Segment VII: Right posterior superior segment
- Segment VIII: Right anterior superior segment
Basis of Liver Segmentation
Functional hepatic segmentation is determined by the internal distribution of the portal triad rather than by the external appearance of the liver.
Branches of the portal vein divide the hepatic parenchyma into territories that receive their own vascular inflow and biliary drainage.
Portal Pedicles
A portal pedicle contains branches of the portal vein, hepatic artery, and bile duct traveling together within connective tissue sheaths.
These structures enter individual hepatic territories and form the anatomical basis of segmental liver organization.
Intrasegmental Structures
Branches of the portal vein, hepatic artery, and biliary tree generally travel within individual hepatic segments.
They are therefore described as intrasegmental structures.
Intersegmental Structures
The major hepatic veins generally travel in planes between portal territories.
They are therefore described as intersegmental structures and provide important landmarks for separating hepatic segments and sectors.
Functional Right and Left Liver
The liver is functionally divided into right and left portions according to the distribution of the right and left portal pedicles.
This functional division does not correspond to the external division produced by the falciform ligament.
Main Portal Scissura
The principal plane separating the functional right and left liver extends approximately from the gallbladder fossa anteriorly toward the inferior vena cava posteriorly.
The middle hepatic vein lies within or close to this plane.
Cantlie Line
The approximate surface projection of the functional right-left hepatic division is traditionally called Cantlie's line.
It extends from the region of the gallbladder fossa toward the inferior vena cava.
Falciform Ligament and Functional Division
The falciform ligament separates the anatomical right and left lobes on the diaphragmatic surface but does not correspond to the main internal vascular division of the liver.
As a result, part of the liver located anatomically to the right of the falciform ligament belongs functionally to the left liver.
Right and Left Portal Veins
Near the porta hepatis, the portal vein usually divides into right and left portal veins.
These vessels supply the functional right and left hepatic territories and subsequently divide into sectoral and segmental branches.
Right Portal Vein
The right portal vein is generally shorter than the left and commonly divides into anterior and posterior sectoral branches.
These branches supply the anterior and posterior sectors of the functional right liver.
Right Anterior Sector
The right anterior sector consists principally of segments V and VIII.
Segment V occupies the inferior portion of this sector, while segment VIII occupies the superior portion.
Right Posterior Sector
The right posterior sector consists principally of segments VI and VII.
Segment VI is inferior, while segment VII is superior and posterior.
Left Portal Vein
The left portal vein has a longer and more complex course than the right portal vein.
Its branches supply segments II, III, and IV and may contribute to the vascular supply of the caudate segment.
Left Lateral Sector
The left lateral sector consists of segments II and III.
Segment II is predominantly superior and posterior, while segment III is predominantly inferior and anterior.
Left Medial Territory
Segment IV forms the principal left medial territory.
It lies between the middle hepatic vein on its right and the plane associated with the left hepatic venous system on its left.
Segment I
Segment I corresponds to the caudate lobe and occupies a distinctive posterior position adjacent to the inferior vena cava.
It differs from the other segments because it may receive portal venous and hepatic arterial branches from both right and left hepatic systems.
Location of Segment I
Segment I lies posteriorly, between the inferior vena cava and the fissure for the ligamentum venosum.
It is situated posterior to the porta hepatis.
Vascular Supply of Segment I
The caudate segment may receive multiple small portal venous and arterial branches from both sides of the hepatic hilum.
This variable bilateral inflow contributes to its relative functional independence.
Venous Drainage of Segment I
Segment I commonly drains directly into the inferior vena cava through multiple small caudate veins.
It therefore does not depend entirely on the major right, middle, or left hepatic veins for venous drainage.
Clinical Importance of Segment I
The distinctive vascular anatomy and deep posterior location of segment I are important in hepatic surgery and imaging.
Its direct relationship with the inferior vena cava can make isolated caudate resection technically challenging.
Segment II
Segment II forms the superior portion of the left lateral hepatic sector.
It occupies the superior and posterior part of the functional left lateral liver.
Location of Segment II
Segment II lies predominantly superior to the left portal vein and lateral to the left hepatic venous plane.
It extends toward the left lateral hepatic margin.
Portal Supply of Segment II
Segment II receives a segmental portal branch from the left portal venous system.
Corresponding hepatic arterial and biliary branches accompany this portal branch.
Segment III
Segment III forms the inferior portion of the left lateral hepatic sector.
It lies inferior to segment II and extends toward the inferior and anterior aspect of the left liver.
Location of Segment III
Segment III lies predominantly inferior to the left portal vein.
It is positioned lateral to segment IV and forms much of the inferior left lateral hepatic region.
Segment II Versus Segment III
| Feature | Segment II | Segment III |
|---|---|---|
| Sector | Left lateral | Left lateral |
| Vertical position | Superior | Inferior |
| General orientation | More posterior | More anterior and inferior |
| Portal source | Left portal vein | Left portal vein |
Segment IV
Segment IV forms the left medial hepatic territory.
It lies functionally within the left liver even though much of it is located anatomically to the right of the falciform ligament.
Relationship to the Quadrate Lobe
The traditional quadrate lobe corresponds principally to the inferior portion of segment IV.
This relationship demonstrates the difference between traditional surface anatomy and functional hepatic segmentation.
Subdivision of Segment IV
Segment IV is commonly subdivided into:
- Segment IVa: Superior portion
- Segment IVb: Inferior portion
Segment IVa
Segment IVa is the superior part of segment IV and lies above the level of the portal venous plane.
It occupies a central superior position within the functional left liver.
Segment IVb
Segment IVb is the inferior portion of segment IV.
It includes much of the region traditionally described externally as the quadrate lobe.
Segment V
Segment V forms the inferior part of the right anterior sector.
It lies immediately to the right of the principal functional plane separating the right and left liver.
Location of Segment V
Segment V is positioned inferiorly and anteriorly within the functional right liver.
It has an important relationship with the gallbladder fossa.
Portal Supply of Segment V
Segment V receives branches from the right anterior portal pedicle.
Associated hepatic arterial and biliary branches accompany the portal venous supply.
Segment VI
Segment VI forms the inferior portion of the right posterior sector.
It lies posterior and lateral to segment V.
Location of Segment VI
Segment VI occupies the inferior-posterior portion of the right liver.
It extends toward the inferior and lateral hepatic margin.
Portal Supply of Segment VI
Segment VI receives a branch of the right posterior portal pedicle.
Its corresponding arterial and biliary branches travel with the portal venous branch.
Segment VII
Segment VII forms the superior portion of the right posterior sector.
It is located high in the posterior right liver near the diaphragm.
Location of Segment VII
Segment VII lies superior to segment VI and posterior to segment VIII.
Its deep superior-posterior location can make it relatively difficult to access surgically.
Relations of Segment VII
Segment VII is closely related to the diaphragm and posterior abdominal structures.
It lies near the posterior hepatic surface and the retrohepatic inferior vena cava.
Segment VIII
Segment VIII forms the superior portion of the right anterior sector.
It lies superior to segment V and anterior to segment VII.
Location of Segment VIII
Segment VIII occupies a superior and relatively central portion of the right hepatic parenchyma.
It extends toward the diaphragmatic surface.
Portal Supply of Segment VIII
Segment VIII receives branches from the right anterior portal pedicle.
Its portal supply is distinct from the posterior sectoral supply of segments VI and VII.
Segments V and VIII
Segments V and VIII together form the right anterior sector.
The right anterior portal pedicle provides their principal portal inflow.
Segments VI and VII
Segments VI and VII together form the right posterior sector.
They receive their principal portal inflow from the right posterior portal pedicle.
Segmental Arrangement
| Segment | General Location | Functional Territory |
|---|---|---|
| I | Posterior, caudate | Functionally distinctive caudate segment |
| II | Left lateral superior | Left lateral sector |
| III | Left lateral inferior | Left lateral sector |
| IV | Left medial | Left medial territory |
| V | Right anterior inferior | Right anterior sector |
| VI | Right posterior inferior | Right posterior sector |
| VII | Right posterior superior | Right posterior sector |
| VIII | Right anterior superior | Right anterior sector |
Clockwise Numbering
When viewed in an axial orientation from below, the segments are often conceptualized as being arranged approximately clockwise around the portal structures, beginning with the left lateral segments and continuing through the right liver.
Segment I remains distinctive because of its posterior position.
Portal Vein as a Horizontal Landmark
The right and left portal veins provide an important approximately horizontal reference for separating superior and inferior hepatic segments.
Segments II, IVa, VII, and VIII are generally superior, while segments III, IVb, VI, and V are generally inferior.
Superior Hepatic Segments
- Segment II
- Segment IVa
- Segment VII
- Segment VIII
Inferior Hepatic Segments
- Segment III
- Segment IVb
- Segment V
- Segment VI
Hepatic Veins
The major hepatic veins are critical landmarks in segmental liver anatomy.
The right, middle, and left hepatic veins generally travel in planes separating major portal territories and drain directly into the inferior vena cava.
Right Hepatic Vein
The right hepatic vein lies between the right anterior and right posterior portal sectors.
It therefore forms an important plane separating segments V and VIII anteriorly from segments VI and VII posteriorly.
Middle Hepatic Vein
The middle hepatic vein lies near the principal functional division between the right and left liver.
It separates segments V and VIII on the right from segment IV on the left.
Left Hepatic Vein
The left hepatic vein forms an important plane within the functional left liver.
Its relationship helps separate the left lateral territory from the more medial hepatic parenchyma.
Major Hepatic Venous Planes
| Vein | Principal Segmental Relationship |
|---|---|
| Right hepatic vein | Separates right anterior and posterior sectors |
| Middle hepatic vein | Separates functional right and left liver |
| Left hepatic vein | Helps separate left lateral and medial territories |
Portal Veins Versus Hepatic Veins
| Feature | Portal Veins | Hepatic Veins |
|---|---|---|
| Relationship to segments | Travel within segments | Generally travel between segments or sectors |
| Function | Provide portal inflow | Provide venous outflow |
| Surgical role | Define portal territories | Define intersegmental planes |
| Major termination | Hepatic sinusoids | Inferior vena cava |
Hepatic Arterial Supply
Each segment receives arterial branches that generally accompany its portal venous branches.
The hepatic arterial anatomy can vary substantially, but the segmental branches ultimately distribute oxygenated blood to individual portal territories.
Biliary Drainage
Each hepatic segment has biliary branches that generally accompany the portal venous and hepatic arterial branches.
These ducts converge progressively into larger intrahepatic ducts and ultimately contribute to the right and left hepatic ducts.
Segmental Independence
The combination of relatively independent portal inflow, arterial supply, and biliary drainage allows individual segments or groups of segments to function as anatomical units.
This organization is the foundation of modern anatomical liver resection.
Portal Triads Within the Liver
Branches of the portal vein, hepatic artery, and bile ducts travel together within connective tissue extensions of the hepatic fibrous capsule.
These structures progressively branch as they enter smaller functional territories.
Glissonian Pedicles
The portal triad branches surrounded by connective tissue are often referred to surgically as Glissonian pedicles.
Control of selected pedicles can isolate the vascular inflow and biliary drainage of particular hepatic territories.
Anatomical Lobes Versus Segments
The traditional anatomical lobes of the liver are defined primarily by external landmarks such as the falciform ligament and fissures of the visceral surface.
The Couinaud segments are defined by internal vascular and biliary anatomy and are therefore more useful for surgical planning.
Anatomical and Functional Division
| Feature | Anatomical Lobes | Functional Segments |
|---|---|---|
| Basis | Surface morphology | Portal and biliary distribution |
| Main landmarks | Falciform ligament, fissures and fossae | Portal branches and hepatic veins |
| Number | Traditionally four lobes | Eight Couinaud segments |
| Primary clinical use | Gross anatomical description | Imaging and hepatic surgery |
Relationship to the Falciform Ligament
The falciform ligament does not divide the liver into functional right and left halves.
Segments III and IV lie on opposite sides of the falciform ligament but both belong to the functional left liver.
Relationship to the Gallbladder
The gallbladder fossa lies near the functional division between the right and left liver.
Segment IV lies predominantly medial to the gallbladder fossa, while segment V lies predominantly lateral to it.
Relationship to the Inferior Vena Cava
The inferior vena cava lies posterior to the liver and receives the major hepatic veins.
Segment I is closely related to the IVC and frequently drains directly into it through short hepatic veins.
Relationship to the Porta Hepatis
The porta hepatis is the entry and exit region for the major portal, arterial, and biliary structures.
Division of these structures near the hepatic hilum gives rise to the branches supplying the functional hepatic territories.
Segmental Anatomy on Ultrasound
Ultrasound uses portal and hepatic veins as internal landmarks for identifying hepatic regions.
Portal veins can often be distinguished by their echogenic walls, while hepatic veins converge toward the inferior vena cava.
Segmental Anatomy on CT
Contrast-enhanced CT provides detailed visualization of portal and hepatic venous anatomy.
The location of a hepatic lesion can be assigned to a segment by evaluating its relationship to portal branches, hepatic veins, and other internal landmarks.
Segmental Anatomy on MRI
MRI provides detailed visualization of hepatic parenchyma, vessels, and biliary structures.
Segmental localization is particularly important when characterizing lesions and planning surgical or interventional treatment.
Why Segmental Localization Matters
Describing a hepatic lesion simply as being in the right or left lobe may provide insufficient anatomical information.
Segmental localization identifies its relationship to vascular territories and helps determine which vessels, ducts, and hepatic regions may be involved.
Hepatic Resection
The functional independence of hepatic segments permits anatomical liver resection.
A surgeon may remove a segment, sector, section, or larger functional portion while preserving viable hepatic tissue supplied by other portal pedicles.
Segmentectomy
A segmentectomy removes an individual functional hepatic segment.
The procedure requires identification and control of the relevant portal pedicle while preserving vascular and biliary structures serving neighboring segments.
Sectionectomy
Groups of adjacent segments sharing major portal branches can be removed together.
Examples include left lateral sectionectomy involving segments II and III and right posterior sectionectomy involving segments VI and VII.
Left Lateral Sectionectomy
A left lateral sectionectomy removes segments II and III.
These segments form a relatively discrete portal territory and are commonly considered together in hepatic surgery.
Right Anterior Sectionectomy
A right anterior sectionectomy involves removal of segments V and VIII.
These segments share the right anterior portal pedicle.
Right Posterior Sectionectomy
A right posterior sectionectomy removes segments VI and VII.
These segments share the right posterior portal pedicle.
Right Hepatectomy
A functional right hepatectomy generally removes segments V, VI, VII, and VIII.
The operation follows the functional right-left division rather than the falciform ligament.
Left Hepatectomy
A functional left hepatectomy generally removes segments II, III, and IV.
Segment I may be preserved or removed depending on the indication and extent of surgery.
Extended Hepatic Resections
More extensive operations may remove additional segments across the principal functional division.
Precise knowledge of portal and hepatic venous anatomy is essential for preserving adequate functional liver remnant.
Caudate Lobectomy
Isolated resection of segment I is technically demanding because of its deep location and intimate relationship with the inferior vena cava and hepatic hilum.
Multiple small caudate veins may require individual control.
Liver Transplantation
Segmental anatomy is fundamental to liver transplantation, particularly living-donor transplantation.
Donor and recipient anatomy must be assessed carefully to identify portal venous, arterial, hepatic venous, and biliary branching patterns.
Living-Donor Transplantation
Living-donor grafts may consist of the right liver, left liver, or left lateral section depending on recipient requirements and donor anatomy.
Understanding segmental vascular territories is essential for preserving adequate liver volume in both donor and recipient.
Portal Vein Embolization
Portal vein embolization can be used before selected major hepatic resections to redirect portal flow toward the portion of liver that will remain.
This can stimulate hypertrophy of the future liver remnant before surgery.
Interventional Radiology
Segmental hepatic anatomy is important for image-guided interventions such as tumor ablation, arterial embolization, portal venous procedures, and percutaneous biliary interventions.
Precise localization helps target diseased tissue while limiting injury to adjacent hepatic territories.
Hepatic Tumors
Primary and metastatic hepatic tumors are routinely described according to their segmental location.
The relationship of a tumor to portal pedicles and hepatic veins influences resectability and operative planning.
Hepatocellular Carcinoma
Segmental localization of hepatocellular carcinoma helps determine possible surgical, ablative, and catheter-based approaches.
Imaging also evaluates involvement of portal and hepatic venous structures.
Metastatic Disease
Liver metastases may involve one or multiple hepatic segments.
Mapping their segmental distribution helps determine whether anatomical resection, limited resection, ablation, or another treatment strategy is technically feasible.
Segmental Biliary Obstruction
Obstruction of an intrahepatic bile duct can produce dilatation within the hepatic territory drained by that duct.
Understanding segmental biliary anatomy helps localize the level of obstruction.
Segmental Vascular Variations
The Couinaud system provides a useful standard framework, but individual portal, arterial, hepatic venous, and biliary anatomy varies.
Preoperative imaging is therefore necessary before complex hepatic procedures rather than relying solely on textbook branching patterns.
Portal Vein Variations
Variations include portal trifurcation, early origin of the right posterior portal branch, and other alternative branching configurations.
These patterns can alter the arrangement of portal pedicles used during surgery.
Hepatic Vein Variations
Accessory hepatic veins are common, particularly in the right liver.
Inferior right hepatic veins may provide significant venous drainage and can be important during hepatic resection or transplantation.
Biliary Variations
Segmental bile ducts show substantial variation, particularly in the right liver.
Some ducts may cross conventional sectoral boundaries before joining the main biliary system.
Arterial Variations
Segmental hepatic arterial branches may arise from conventional, replaced, or accessory hepatic arteries.
Detailed vascular mapping is especially important before transplantation and complex liver surgery.
Summary of Couinaud Segments
| Segment | Location | Major Group |
|---|---|---|
| I | Caudate, posterior | Caudate segment |
| II | Left lateral superior | Left lateral sector |
| III | Left lateral inferior | Left lateral sector |
| IVa | Left medial superior | Left medial territory |
| IVb | Left medial inferior | Left medial territory |
| V | Right anterior inferior | Right anterior sector |
| VI | Right posterior inferior | Right posterior sector |
| VII | Right posterior superior | Right posterior sector |
| VIII | Right anterior superior | Right anterior sector |
Key Segmental Relationships
| Relationship | Segments |
|---|---|
| Left lateral sector | II and III |
| Left medial territory | IV |
| Right anterior sector | V and VIII |
| Right posterior sector | VI and VII |
| Caudate segment | I |
| Functional right liver | V, VI, VII and VIII |
| Functional left liver | II, III and IV |
Key Features of Liver Segmentation
| Feature | Key Point |
|---|---|
| Classification | Couinaud system |
| Number of principal segments | Eight |
| Segment I | Caudate segment |
| Segment IV | Left medial territory, commonly divided into IVa and IVb |
| Portal triad branches | Generally intrasegmental |
| Hepatic veins | Generally intersegmental |
| Right-left functional plane | Approximately gallbladder fossa to IVC |
| Major clinical importance | Imaging, liver resection, transplantation and interventional procedures |
Anatomical and Clinical Importance
The Couinaud classification provides a functional map of the liver based on its internal vascular and biliary organization. Unlike the traditional anatomical lobes, which are defined mainly by surface landmarks, hepatic segments correspond to portal territories with relatively independent vascular inflow and biliary drainage.
The portal vein and its accompanying hepatic arterial and biliary branches travel within these territories, while the major hepatic veins generally occupy planes between them. The right hepatic vein separates the right anterior and posterior sectors, the middle hepatic vein lies near the functional right-left division, and the left hepatic vein helps define territories within the left liver.
This organization has major clinical importance because liver disease can be localized precisely and selected segments can be surgically removed while preserving functioning hepatic tissue. Segmental anatomy therefore forms the foundation for modern interpretation of hepatic imaging, anatomical liver resection, transplantation, tumor localization, and many interventional radiological procedures.
Published on October 1, 2026
Last updated on October 1, 2026
Last updated on October 1, 2026