Liver

The liver is the largest abdominal organ that plays a major role in metabolism and has a number of functions, including glycogen storage, decomposition of red blood cells, plasma protein synthesis, hormone production, and detoxification. It is one of the very few organs that has the ability to regenerate.

Gross anatomy

The liver is an irregular, wedge-shaped organ that lies below the diaphragm in the right upper quadrant of the abdominal cavity and is in close approximation with the diaphragm, stomach and the gallbladder. It is largely covered by the costal cartilages.

The liver is made of several functional units called lobules, which in turn can be subdivided into smaller units called sinusoids. Each lobule — i.e. the entire liver, is covered by a fibrous capsule known as Glisson's capsule, which forms the innermost liver coverage.

The liver is almost entirely covered by visceral peritoneum and is associated with a number of peritoneal ligaments including the falciform ligament. The posterocranial aspect of the liver, adjacent to the dorsal body wall, is not completely covered by peritoneum, resulting in the so-called "bare area" (area nuda). The 

The liver is described as having two surfaces, diaphragmatic and visceral, sharply demarcated anteriorly by the inferior margin:

• diaphragmatic surface: smooth peritoneal area that faces superiorly and anteriorly and includes the bare area
• visceral surface: faces inferiorly and posteriorly and is covered by peritoneum ⁶

The liver usually measures ⁷: 

• craniocaudal length: 10-12.5 cm
• transverse diameter: 20-23 cm

Segmental anatomy

Traditionally, the liver was divided into four anatomical lobes. However, this has been superseded by the use of the Couinaud classification which divides the liver into eight functional units (known as segments), supplied by individual segmental hepatic arteries, portal veins and bile ducts, which can be individually resected.

Blood supply

The liver receives a dual blood supply from the portal vein and hepatic arteries. The hepatic portal vein supplies ~75% of the liver's blood supply by volume and carries venous blood drained from the spleen, gastrointestinal tract, and its associated organs (hence oxygen-poor and nutrient-rich).

The hepatic arteries supply arterial blood to the liver and account for the remainder of its blood flow (hence oxygen-rich and nutrient-poor). The hepatic arterial system supplies the biliary system.

Oxygen is provided from both sources; approximately half of the liver's oxygen demand is met by the hepatic portal vein, and half is met by the hepatic arteries.

Lymphatics

The majority of the lymph from the liver drains into nodes that lie in the porta hepatis. Drainage channels of these lymph nodes follow the hepatic artery to reach the retropyloric and then the coeliac lymph nodes. The superior surface of the liver also has communications with extraperitoneal lymphatics that perforate the diaphragm and drain into mediastinal lymph nodes.

Innervation

The liver is supplied by sympathetic and parasympathetic fibres from the hepatic nerve plexus, which travel with branches of the hepatic artery and portal vein to the liver. Within the liver, the nerve fibres accompany the portal triad. Sympathetic fibres are derived from the coeliac plexus and parasympathetic fibres are derived from the anterior and posterior vagal trunks ⁵. 

Variant anatomy

• lobar variation
  • Riedel lobe
  • agenesis of the right hepatic lobe
  • agenesis of the left hepatic lobe
  • accessory hepatic lobes / ectopic liver tissue
  • beaver tail liver
  • supradiaphragmatic liver
• pseudolipoma of the Glisson capsule
• accessory hepatic sulci