Liver Anatomy: Lobes and Hepatic Segments

• The liver, the largest internal organ, resides in the upper right quadrant of the abdomen, under the diaphragm.
• Key functions include metabolism, detoxification, plus vitamin and mineral storage.
• The liver is divided anatomically into lobes, segments, and zones, each having distinct features and functions

Lobes of the Liver

• Traditional anatomy recognizes two major lobes: right and left.
• The falciform ligament separates these lobes and attaches the liver to the anterior abdominal wall.
• The right lobe is much larger, making up about two-thirds of the liver's mass.
• Two smaller lobes also exist: caudate and quadrate.
• The caudate lobe sits on the posterior surface, near the inferior vena cava.
• The quadrate lobe is on the inferior surface, between the gallbladder and the falciform ligament.

Hepatic Segments

• Couinaud's segments are a clinically relevant liver division.
• Claude Couinaud's system divides the liver into eight independent segments, each with its own vascular inflow, biliary drainage, and lymphatic drainage.
• Segments are numbered I-VIII, typically counter-clockwise from an inferior view.
• Segment I is the caudate lobe, receiving blood from both right and left hepatic arteries.
• Segments II and III are in the left lobe, lateral to the falciform ligament.
• Segment IV, also in the left lobe and medial to the falciform ligament, includes the quadrate lobe.
• Segments V-VIII are in the right lobe.
• The right, middle, and left hepatic veins separate these segments.
• Hepatic segments are surgically important.
• Each segment can be independently resected.

Liver Vasculature

• The liver has a dual blood supply from the hepatic artery and portal vein.
• The hepatic artery delivers oxygenated blood from the aorta, about 25% of the liver's blood supply.
• The portal vein carries nutrient-rich blood from digestive organs, the spleen, and the pancreas, providing the other 75%.
• The hepatic artery and portal vein branch into hepatic sinusoids inside the liver.
• These sinusoids are specialized capillaries, allowing close contact between hepatocytes and blood.
• Blood from sinusoids drains into central veins, which form the hepatic veins.
• Hepatic veins empty into the inferior vena cava, returning blood to systemic circulation.

Microscopic Structure of the Liver

• Hepatocytes, specialized liver cells, are arranged in hexagonal lobules.
• A central vein, which drains blood from the sinusoids, is at each lobule's center.
• Plates of hepatocytes surround the central vein, separated by hepatic sinusoids.
• Sinusoids are lined with endothelial cells and Kupffer cells, macrophages removing bacteria and debris.
• The space of Disse, between hepatocytes and endothelial cells, contains blood plasma.
• This space allows substance exchange between hepatocytes and the bloodstream.
• Bile canaliculi, small channels between hepatocytes, collect bile.
• These merge into larger bile ducts, draining into the common hepatic duct.
• The common hepatic duct joins the cystic duct from the gallbladder to form the common bile duct.
• The common bile duct empties into the duodenum, where bile aids in fat digestion and absorption.

Portal Triad

• Portal triads at each liver lobule's corners include a hepatic artery branch, a portal vein branch, and a bile duct.
• These structures supply and drain the liver lobule, ensuring function and maintenance.
• The hepatic artery provides oxygenated blood, while the portal vein delivers nutrient-rich blood from the digestive system.
• The bile duct transports bile to the gallbladder or directly to the duodenum.

Functional Zones of the Liver

• Functional zones divide the liver based on hepatocyte metabolic activity.
• Rappaport's acinus model describes these zones, reflecting oxygen and nutrient gradients from the portal triad to the central vein.
• Zone 1, closest to the portal triad, receives the most oxygenated blood and nutrients.
• Hepatocytes in zone 1 are involved in oxidative metabolism, gluconeogenesis, and bile acid synthesis.
• Zone 2, in the mid-lobular region, has intermediate oxygen and nutrient levels.
• Hepatocytes in zone 2 perform glycolysis, lipogenesis, and drug metabolism.
• Zone 3, closest to the central vein, receives the least oxygenated blood and nutrients.
• Hepatocytes in zone 3 handle glycolysis, lipogenesis, and detoxification.
• Because it has a lower oxygen supply, zone 3 is more susceptible to ischemic injury and steatosis (fatty liver).

Clinical Significance of Liver Anatomy

• Liver anatomy knowledge is essential for surgery, transplantation, and diagnostic imaging.
• Surgeons use hepatic segments to plan resections, minimizing damage to healthy tissue.
• Transplant surgeons use anatomical landmarks for proper placement and vascular connections.
• Radiologists use CT scans and MRIs to visualize the liver and identify abnormalities based on anatomy.
• Understanding functional zones helps clinicians interpret liver enzyme patterns and predict liver damage effects.