Couniaud divided liver into 8 functional segments, each of which is supplied by it’s own portal triad (composed of a portal vein, hepatic artery and a bile duct).
Hepatic veins divide the liver in saggital plane:
1. Middle hepatic vein: Divides the liver into right and left functional lobe.
- Cantle’s line: run from middle of gall bladder fossa anterior to Inferior venacava posteriorly
- Left lobe = Segment I-IV (Segment I is separately the caudate lobe)
- Right lobe = Segment V-VIII
2. Left hepatic vein: Divides left lobe into lateral and medial segments.
- Lateral segment = Segment II and III
- Medial segment = Segment IV
Falciform ligament: left hepatic vein is located slightly left to the left hepatic vein; hence, falciform ligament roughly divides liver into right and left lobe.
3. Right hepatic vein: Divides right lobe into anterior and posterior segments.
- Anterior segment = Segment V and VIII
- Posterior segment = Segment VI and VII
Portal vein divides the liver in transverse plane:
- Upper segment = Segment II, IVa, VIII, VII
- Lower segment = Segment III, IVb, V, VI
How to remember the orientation of Couniaud Liver Segments?
We have a handy hand mnemonic for this purpose 1. Also, remember that the segments are numbered in a clock-wise fashion.
With the right hand, make a fist while tucking the thumb behind the remainder of the fingers.
- Tucked in thumb = Segment I (Caudate Lobe)
- Line of Proximal Interphalangeal (PIP) joints = Plane of portal vein, separating liver into upper and lower segments.
- Interdigitary spaces = Intersegmental planes (hepatic fissures – left, middle and right) in which corresponding hepatic veins run and divide liver sagitally
For the remaining 7 segmets, go in a clockwise direction starting from the lateralmost upper left part, i.e. Proximal phalax of 1st finger.
- Proximal phalanx of 1st finger = Left superior lateral segment = Segment II
- Distal phalanx of 1st finger = Left inferior lateral segment = Segment III
- 2nd finger = Left medial segment = Segment IV (Quadrate lobe)
- Proximal phalanx = Segment IVa
- Distal phalanx = Segment IVb
- Distal phalanx of 3rd finger = Right inferior anterior segment = Segment V
- Distal phalanx of 4th finger = Right inferior posterior segment = Segment VI
- Proximal phalanx of 4th finger = Right superior posterior segment = Segment VII
- Proximal phalanx of 3rd finger = Right superior anterior segment = Segment VIII
Segmental Anatomy of Liver
The left portal vein supplies Couinaud segments I, II, III, and IV.
The right portal vein subdivides into anterior and posterior branches.
- Anterior branch: supplies segments V and VIII
- Posterior branch: supplies segments VI and VII.
Right hepatic vein: drains all of segment VI and VII and some of segments V and VIII.
Left hepatic vein: drains segments II and III and IV.
Middle hepatic vein: drains segment IV, V and VIII.
Intrahepatic bile ducts:
Segment II-IV: Left hepatic duct
Segment V and VIII: Right anterior hepatic duct
Segment VI and VII: Right posterior hepatic duct
Caudate lobe Segment I):
The left lobe is supplied or drained by left branches from porta hepatis components and right from right. The caudate lobe is anatomically different from other lobes:
- It hase direct connections to the IVC through hepatic veins
- It may be supplied by both right and left branches of the portal vein.
- It is drained drained by both right and left hepatic ducts.
Caudate lobe is the lobe that is not affected in Budd-Chiari syndrome and cirrhosis of liver because of it’s direct drainage to IVC.
Hepatectomy and Sectionectomy
Right hepatectomy = Segment V-VIII (± segment I)
Extended right hepatectomy or Right trisectionectomy = Right hepatectomy + Segment IV
Left hepatectomy = Segment II-IV (± segment I)
Extended left hepatectomy or Left trisectionectomy = Left hepatectomy + Segment V and VIII
Right posterior sectionectomy = segment VI and VII
Right anterior sectionectomy = segment V and VIII
Left medial sectionectomy = segment IV
Left lateral sectionectomy = segment II and III