Anatomy of liver

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Anatomy of the Liver

Overview

The liver is the largest solid organ and the heaviest gland in the body, weighing approximately 1500 g in adults. It is reddish-brown, resides in the right upper quadrant beneath the diaphragm, and is protected by the lower rib cage. The entire organ is surrounded by a fibrous sheath called Glisson's capsule.

Ligaments and Peritoneal Attachments

The liver is held in position by several ligaments derived from peritoneum:

Figure: Hepatic ligaments - falciform, round, and triangular ligaments (Schwartz's Principles of Surgery)

Ligament	Description
Falciform ligament	Connects liver to anterior abdominal wall; separates the left lateral segment from the left medial segment along the umbilical fissure
Round ligament (ligamentum teres)	Remnant of the obliterated umbilical vein; runs along the inferior edge of the falciform ligament into the left liver hilum
Ligamentum venosum (Arantius' ligament)	Obliterated ductus venosus; lies deep in the plane between caudate lobe and left lateral segment
Left & right triangular ligaments	Secure the two sides of the liver to the diaphragm
Coronary ligaments	Extend anteriorly from the triangular ligaments; the right coronary also anchors the liver to the right retroperitoneum overlying the right kidney
Hepatoduodenal ligament (porta hepatis)	Connects liver to superior duodenum; contains the common bile duct, hepatic artery, and portal vein
Gastrohepatic ligament	Connects liver to lesser curvature of stomach

The foramen of Winslow (epiploic foramen) lies posterior to the porta hepatis and connects to the lesser sac - it allows complete vascular inflow control by clamping the hepatoduodenal ligament (the Pringle maneuver).

Lobes - Anatomical vs. Functional

Anatomical (Surface) Lobes

Grossly, the liver has four anatomical lobes:

Right lobe - the largest
Left lobe - separated from the right by the falciform ligament
Caudate lobe - on the posterior-inferior surface
Quadrate lobe - on the inferior surface, between the gallbladder fossa and the round ligament

Functional (Surgical) Division

The true functional division is defined by Cantlie's line - an imaginary plane running from the gallbladder fossa to the inferior vena cava (IVC). The middle hepatic vein runs in this plane. The right functional lobe accounts for ~60-70% of the liver mass, while the left lobe (plus caudate) makes up the remainder. The falciform ligament does not separate the right and left functional lobes.

Segmental Anatomy (Couinaud's Segments)

The most important advance in understanding liver anatomy came from the cast-work studies of the French surgeon Couinaud in the early 1950s. He divided the liver into 8 segments, numbered in a clockwise direction with the caudate lobe as segment I.

Couinaud's liver segments - anterior and posterior views

Figure: Couinaud's liver segments I through VIII, anterior (top) and inferior (bottom) views (Schwartz's Principles of Surgery)

Segment	Location	Notes
I	Caudate lobe	Unique - drains directly into IVC
II & III	Left lateral segment	Left lobe, lateral portion
IV (IVa & IVb)	Left medial segment	IVa is cephalad (near MHV); IVb is caudad (quadrate lobe)
V & VIII	Right anterior lobe	V is inferior; VIII is superior
VI & VII	Right posterior lobe	VI is inferior; VII is superior

Left lobe = segments II, III, IV
Right lobe = segments V, VI, VII, VIII
A right hepatectomy removes segments V-VIII; a left hepatectomy removes segments II-IV

Bismuth further described four sectors based on hepatic vein distribution, each running in a fissura (fissure): right hepatic vein divides right posterolateral from right anterolateral sector; middle hepatic vein separates right and left livers; left hepatic vein separates left posterior from left anterior sectors.

Blood Supply

The liver has a dual blood supply:

Source	Contribution	Origin
Portal vein	~75% of blood supply	Confluence of splenic vein + superior mesenteric vein (SMV)
Hepatic artery	~25% of blood supply	Celiac axis (trunk)

Hepatic Artery

The celiac trunk gives off the left gastric, splenic, and common hepatic arteries. The common hepatic artery divides into:

Gastroduodenal artery
Hepatic artery proper → divides into right and left hepatic arteries

The "classic" anatomy is present in only ~76% of cases. Important variants:

Replaced/accessory right hepatic artery from SMA: 10-15% of cases
Replaced/accessory left hepatic artery from left gastric artery: 3-10%
Both replaced right + left: ~1-2%
Completely replaced common hepatic from SMA: ~1-2%

The cystic artery (to gallbladder) typically arises from the right hepatic artery within Calot's triangle. The right hepatic artery passes posterior to the common bile duct ~88% of the time.

Portal Vein

The main portal vein travels within the porta hepatis before dividing into left and right branches. The left portal vein has a characteristic transverse portion followed by a 90° turn at the umbilical fissure to become the umbilical portion. The caudate lobe receives portal branches from both right and left main portal veins. Portal vein trifurcation or aberrant branching occurs in 20-35% of individuals.

Venous Drainage

Three hepatic veins drain blood into the suprahepatic IVC:

Vein	Drains
Right hepatic vein	Segments V, VI, VII, VIII
Middle hepatic vein	Segment IV + portions of V and VIII
Left hepatic vein	Segments II and III

The caudate lobe (segment I) drains directly via its own small veins directly into the IVC - this is what makes it unique.

The left and middle hepatic veins form a common trunk ~95% of the time before entering the IVC; the right hepatic vein inserts separately. A large inferior accessory right hepatic vein is present in 15-20% of cases.

Biliary System

Within the liver, bile flows from: Bile canaliculi (between hepatocytes) → bile ducts → right and left hepatic ducts → common hepatic duct

The common hepatic duct is joined by the cystic duct from the gallbladder to form the bile duct (common bile duct). The common bile duct lies anteriorly and to the right within the hepatoduodenal ligament. It descends posterior to the superior duodenum and joins the pancreatic duct to enter the descending duodenum at the major duodenal papilla (of Vater).

The gallbladder sits adherent to hepatic segments IVb and V. Biliary duct variants exist in 30-40% of cases.

Microanatomy - The Hepatic Lobule and Acinus

The liver is composed of 50,000-100,000 discrete anatomic units called lobules.

Figure: The hepatic lobule showing sinusoids, central vein, portal triads, bile canaliculi, space of Disse, and Kupffer cells (Morgan & Mikhail's Clinical Anesthesiology)

Hepatic Lobule

Hepatocytes are arranged in plates radiating cylindrically around a central centrilobular vein
4-5 portal tracts surround each lobule, each containing: hepatic arteriole, portal venule, bile ductule, lymphatics, and nerves (collectively the portal triad)
Blood flows inward from portal tracts → through sinusoids → to the central vein

Hepatic Acinus (Functional Unit)

The acinus (not the lobule) is considered the true functional unit:

A portal tract sits at the center; centrilobular veins are at the periphery
Zone 1 (periportal) - cells closest to portal tract; best oxygenated; first to regenerate after injury
Zone 2 (midzonal)
Zone 3 (centrilobular) - farthest from portal tract; least oxygenated; most susceptible to ischemic injury and toxic drug injury (e.g., acetaminophen toxicity)

Sinusoids

Lie between hepatocyte plates
Lined by endothelial cells and Kupffer cells (resident hepatic macrophages)
Kupffer cells remove bacterial endotoxins, viruses, proteins, and particulate matter
The space of Disse lies between sinusoidal capillaries and hepatocytes - this is where hepatic lymph forms

Innervation and Lymphatics

Sympathetic: T6-T11 fibers
Parasympathetic: Left vagus (anterior hepatic branch) and right vagus (posterior hepatic branch)
Right phrenic nerve also contributes (explains referred shoulder pain with liver pathology)
Lymphatics: An extensive network forms within hepatocyte plates, communicating directly with the space of Disse; hepatic lymph drains to the celiac lymph nodes

Summary Table

Feature	Details
Weight	~1500 g
Anatomical lobes	Right, Left, Caudate, Quadrate
Functional segments	8 (Couinaud)
Arterial supply	Hepatic artery (25%) + Portal vein (75%)
Venous drainage	Right, Middle, Left hepatic veins → IVC
Functional unit	Hepatic acinus (3 zones)
Structural unit	Hepatic lobule (50,000-100,000)
Phagocytic cells	Kupffer cells (sinusoid lining)
Capsule	Glisson's capsule
Key surgical landmark	Cantlie's line (gallbladder fossa → IVC)

Sources:

Schwartz's Principles of Surgery, 11th Edition, Chapter 31
Morgan & Mikhail's Clinical Anesthesiology, 7th Edition, Chapter 33
Gray's Anatomy for Students, Chapter 4
Yamada's Textbook of Gastroenterology, 7th Edition, Chapter 10
Bailey and Love's Short Practice of Surgery, 28th Edition

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