Traditional vs. surgical division
Externally, the liver has two surfaces: the diaphragmatic surface (facies diaphragmatica) and the visceral surface (facies visceralis). Anatomists traditionally divide the organ into the right (lobus dexter) and left (lobus sinister) lobes. On the diaphragmatic surface, they are separated by the falciform ligament, while on the visceral surface, the separation is marked by the fissure for the round ligament and the fissure for the ligamentum venosum. Additionally, the quadrate and caudate lobes are distinguished on the inferior surface.
However, external landmarks are insufficient for the surgeon. In clinical anatomy, the classification proposed by Claude Couinaud in 1957 is used. It relies not on ligaments and fissures, but on the internal vascular architecture.
Two vascular systems of the liver
Within the liver parenchyma, two independent systems of vessels and ducts are distinguished:
- Portal (Glissonian) system. Includes branches of the portal vein, proper hepatic artery, and bile ducts. They enter through the porta hepatis (porta hepatis), and the course of their branches within the organ relatively coincides. This system forms the basis of segmental division.
- Caval system. Includes the hepatic veins, which drain blood into the inferior vena cava. The direction of these vessels does not coincide with the course of the portal system elements.
Along with the portal system vessels, connective tissue penetrates into the organ — the perivascular fibrous capsule (capsula fibrosa perivascularis). Its extensions form a robust connective tissue framework of the liver.
Hierarchy of Couinaud segments
Couinaud's classification groups liver regions radially around the porta hepatis. The basic structure is: 2 lobes $\rightarrow$ 5 sectors $\rightarrow$ 8 segments.
Lobe, sector, and segment are regions of parenchyma possessing absolute functional autonomy. Each of them has isolated blood supply, independent bile drainage, and lymphatic drainage.
Blood enters the liver from two sources: oxygenated blood from the aorta (via the proper hepatic artery) and venous blood, rich in nutrients and toxins, from the unpaired abdominal organs (via the portal vein). In the porta hepatis, the portal vein divides into short right and long left branches, giving rise to segmental branching.
Topography of portal system branching
Segment distribution depends on the order of vascular branching:
- First-order branches supply blood to the right, left, and posterior parts of the liver. It is important to understand that their boundaries do not coincide with external anatomical borders (for example, they run directly through the gallbladder fossa and the groove of the inferior vena cava).
- Second-order branches provide blood flow to four sectors: right medial, right lateral, left medial, and left lateral.
- Formation of segments: the right and left lateral regions are divided into anterior and posterior segments. The left medial region contains only a single segment. The posterior part of the liver, which anatomically corresponds to the caudate lobe, also consists of a single isolated segment.
Porta hepatis — the functional center
All elements of the portal system enter the organ through the porta hepatis (porta hepatis), located on the visceral surface. The porta is the functional analogue of the lung root.
Here are located:
- proper hepatic artery (a. hepatica propria);
- portal vein (v. portae);
- common hepatic duct (ductus hepaticus communis);
- nerves and lymphatic vessels.
These structures approach the porta as part of the hepatoduodenal ligament, where they have a strict topography: the vein lies posteriorly, the artery to the left, and the ducts to the right.
To memorize the topography of elements within the hepatoduodenal ligament (from right to left), use the mnemonic: Duct is to the right, Vein is posterior (in between), Artery is to the left.
Class and exam questions
How does a surgical segment differ from a classical liver lobe?
Classical lobes are separated by external landmarks (e.g., the falciform ligament). A segment, however, is an internal autonomous region with its own blood supply and bile drainage, the boundaries of which are not visible on the organ's surface.
Why is Couinaud's classification based specifically on the portal system?
Branches of the portal vein, hepatic artery, and bile ducts run parallel inside the organ, forming unified vasculo-secretory pedicles. This allows the surgeon to remove a diseased segment by ligating a single pedicle without disrupting the blood supply to adjacent regions.
Do the boundaries of first-order branches coincide with external fissures?
No, the internal blood supply boundaries do not coincide with external fissures. For example, the boundary between the right and left parts of the liver runs through the gallbladder fossa and the groove of the inferior vena cava.
What else to study on this topic
- External structure of the liver: surfaces, margins, and fissure system
- Caval system: blood drainage via hepatic veins
- Liver histology: classical lobule, portal lobule, and acinus
- Metabolic and detoxifying functions of hepatocytes
- Portal hypertension syndrome and hepatocellular insufficiency
- Types of jaundice (suprahepatic, hepatic, subhepatic) and their markers
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