External Structure and Surfaces
Anatomically, the liver is a large solid organ traditionally described as having two main surfaces:
- Diaphragmatic surface (facies diaphragmatica). This surface is smooth, convex, and precisely matches the dome of the diaphragm, against which it firmly rests. It is clearly divided into right and left lobes (lobus hepatis dexter et sinister) by the falciform ligament (lig. falciforme hepatis). Also located here are the coronary ligament (lig. coronarium hepatis) and the paired triangular ligaments (ligg. triangulare dextrum et sinistrum).
- Visceral surface (facies visceralis). Directed inferiorly toward the abdominal viscera, this surface is irregular and marked by impressions from adjacent structures. It features a prominent H-shaped system of fissures and grooves: the groove for the inferior vena cava (sulcus venae cavae), the fissure for the round ligament of the liver (sulcus ligamenti teretis), and the fissure for the ligamentum venosum (sulcus ligamenti venosi).
Porta Hepatis and Visceral Lobes
The central landmark of the visceral surface is the porta hepatis (gate of the liver). This serves as the primary hilum through which major vascular and ductal structures enter and leave the organ.
Structures entering through the porta hepatis include:
- Portal vein (v. portae);
- Proper hepatic artery (a. hepatica propria);
- Autonomic nerve plexuses.
Structures exiting through the porta hepatis include:
- Common hepatic duct (ductus hepaticus communis);
- Lymphatic vessels.
The system of fissures on the visceral surface divides the liver tissue functionally and anatomically into four lobes. In addition to the classic right and left lobes, the quadrate lobe (lobus quadratus) and tail-like or caudate lobe (lobus caudatus) are clearly delineated.
Internal Structure and the Hepatic Lobule
Externally, the organ is covered by a serous coat (tunica serosa), beneath which lies a subserous layer and a dense fibrous capsule (tunica fibrosa). This connective tissue not only envelops the organ but also penetrates its parenchyma via the porta hepatis and the fissure of the round ligament. Accompanying the biliary ducts and blood vessels, it forms the perivascular fibrous capsule (capsula fibrosa perivascularis), creating a robust internal stromal framework.
The structural and functional unit of the liver is the hepatic lobule (lobulus hepaticus). This is a microscopic structure measuring only 1–2 mm. It is formed by hepatic parenchymal cells (hepatocytes) arranged in branching cords known as hepatic plates (laminae hepaticae).
The microcirculation within the lobule is unique. Interlobular arteries and portal venules run in the portal triads at the periphery, giving off branching capillaries that empty into the sinusoids (vasa sinusoidea) running between the hepatic plates. Within these sinusoids, oxygen-rich arterial blood and nutrient-rich venous blood from the portal vein thoroughly mix. This blended blood flows toward the center of the lobule and drains into the central vein (v. centralis). Central veins subsequently empty into sublobular veins, which converge to form the right, intermediate (middle), and left hepatic veins.
Segmental Anatomy of the Liver
The external division into anatomical lobes does not correspond to the internal distribution of blood flow. Modern surgical and clinical anatomy relies on a segmental classification based on the branching patterns of the intrahepatic portal system and biliary ducts.
First-order branches of the portal vein divide the organ into right, left, and posterior divisions. The boundaries between these sectors are internal and project along the gallbladder fossa and the groove for the inferior vena cava.
Second-order branches supply distinct anatomical sectors:
- Right medial and right lateral sectors;
- Left medial and left lateral sectors.
These sectors further subdivide into functional segments (based on Couinaud's classification). The right and left lateral sectors further divide into anterior and posterior segments. The left medial sector consists of a single segment. The posterior region of the organ, anatomically corresponding to the caudate lobe, also constitutes an independent, functionally distinct segment.
Topography and Syntopy
The liver occupies a vast space in the upper right abdominal cavity: it is primarily located in the right hypochondrium, extends across the epigastrium, and its left margin reaches into the left hypochondrium.
Skeletotopy of the liver:
- Superior boundary: On the right midclavicular line, it reaches the level of the 4th intercostal space; on the left, its highest point reaches the 5th intercostal space.
- Inferior border: On the right midclavicular line, it descends to the 10th intercostal space, then follows the right costal margin, crosses the epigastrium, and extends beneath the left costal margin at the level of the 8th left costal cartilage.
- Posterior boundary: On the right scapular line, it projects between the 7th intercostal space and the upper border of the 11th rib.
Syntopy (Relations to adjacent organs): The smooth diaphragmatic surface contacts the right and partially left domes of the diaphragm. The visceral surface relates to numerous organs: the cardia, body, and pylorus of the stomach, the superior part of the duodenum, the right kidney, and the right colic flexure along with the right portion of the transverse colon. Additionally, the gallbladder rests closely against the visceral surface of the right lobe.