Liver Anatomy: Lobes and Segments

• The liver, the largest internal organ in the human body, can be divided into different lobes and segments using both anatomical and functional criteria

Gross Anatomical Lobes

• Traditionally, the liver is divided into two major lobes (right and left) and two minor lobes (caudate and quadrate) based on external anatomical landmarks
• The falciform ligament, a peritoneal fold that attaches the liver to the anterior abdominal wall, is the most obvious external landmark
• It runs on the anterior surface of the liver from the diaphragm to the umbilicus, dividing the liver into right and left lobes
• On the visceral (inferior) surface, the fissure for the ligamentum teres (the obliterated umbilical vein) and the fissure for the ligamentum venosum (the remnant of the fetal ductus venosus) also contribute to this division
• The right lobe is larger than the left lobe and is separated from the left lobe by the falciform ligament anteriorly and the fissure for the ligamentum venosum and teres posteriorly
• The caudate lobe is located on the posterior-superior surface of the liver, between the fissure for the ligamentum venosum and the groove for the inferior vena cava
• It is a small lobe that can be connected to either the right or left lobe, or sometimes considered a separate lobe altogether
• The quadrate lobe is located on the visceral surface, between the gallbladder fossa on the right and the fissure for the ligamentum teres on the left
• It is inferior to the caudate lobe

Functional (Surgical) Lobes

• A more clinically relevant division of the liver is based on its functional independence, determined by the distribution of the hepatic vasculature (portal vein and hepatic artery) and biliary drainage
• This functional division, proposed by Couinaud, divides the liver into two true lobes (right and left) separated by the main portal scissura, an imaginary plane that runs from the gallbladder fossa to the inferior vena cava
• The right and left lobes are functionally independent, each receiving its own primary branch of the portal vein and hepatic artery, and each draining bile via its own hepatic duct
• This division is important for surgical resections, as one lobe can be removed without compromising the function of the other

Hepatic Segments (Couinaud's Segmentation)

• Couinaud further divided the liver into eight functionally independent segments, numbered I to VIII, based on the branching pattern of the portal vein and hepatic veins
• Each segment has its own vascular inflow, biliary drainage, and lymphatic drainage
• Segment I: corresponds to the caudate lobe, which receives blood supply from both the right and left hepatic arteries and portal vein branches
• Segments II and III: located in the left lateral section of the left lobe
• Segment IV: located in the left medial section of the left lobe, further subdivided into IVa (superior) and IVb (inferior)
• Segments V and VI: located in the right anterior section of the right lobe
• Segments VII and VIII: located in the right posterior section of the right lobe
• The hepatic veins (right, middle, and left) run between these segments and drain blood from the liver into the inferior vena cava
• The middle hepatic vein runs in the main plane (portal scissura) between the right and left functional lobes
• This segmentation is crucial for planning liver resections, allowing surgeons to remove specific segments while preserving the function of the remaining liver tissue
• The Couinaud classification is based on the distribution of the portal vein and hepatic veins and divides the liver into eight segments, each with its own vascular inflow, biliary drainage and lymphatic drainage.

Microscopic Anatomy (Histology)

• The liver parenchyma is composed of hepatocytes, which are arranged in plates or cords radiating from a central vein
• Between the plates of hepatocytes are sinusoids, specialized capillaries that receive blood from the hepatic portal vein and hepatic artery
• The sinusoids drain into the central veins, which then coalesce into the hepatic veins
• Kupffer cells, specialized macrophages, are located within the sinusoids and play a role in removing bacteria and debris from the blood
• Bile canaliculi, small channels between hepatocytes, collect bile produced by the hepatocytes
• The bile flows into bile ducts, which eventually merge to form the right and left hepatic ducts
• Portal triads, consisting of a branch of the hepatic artery, portal vein, and bile duct, are located at the periphery of the liver lobules
• The space of Disse is a narrow space between the hepatocytes and the sinusoidal endothelium, containing stellate cells (Ito cells) that store vitamin A

Clinical Significance of Liver Anatomy

• Knowledge of liver anatomy, particularly the functional segmentation, is essential for liver surgery
• Hepatic resections can be performed based on these segments, allowing for removal of diseased tissue while preserving healthy tissue
• Liver transplantation also relies on an understanding of liver anatomy to ensure proper vascular and biliary connections
• Liver imaging techniques, such as CT and MRI, use anatomical landmarks to identify lesions and plan interventions
• Understanding the location of the liver in relation to other abdominal organs is important for diagnosing and managing liver trauma
• Diseases such as cirrhosis and hepatitis can alter the normal liver architecture, affecting its function and blood flow