Anatomy of the Liver

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🧬 Introduction

• The liver is the largest solid organ in the human body (~1.5 kg in adults) and performs >500 vital functions.
• It sits in the right upper quadrant, protected by the rib cage, extending across the midline beneath the diaphragm.
• Functions include metabolism, detoxification, protein synthesis, storage, and bile production.
• A key clinical concept is its segmented anatomy (Couinaud), which guides surgery and transplantation.

📍 Gross Anatomy

Traditionally divided into right and left lobes (falciform ligament as surface landmark), but surgically into 8 Couinaud segments, each with its own vascular inflow, outflow, and biliary drainage → meaning each segment can be resected independently.

• Right Lobe: Larger, subdivided into anterior and posterior sectors.
• Left Lobe: Smaller, subdivided into medial and lateral sectors.

🔢 Surgical Segments (Couinaud Classification)

• Segment I: Caudate lobe — independent blood supply, drains directly into IVC.
• Segment II: Left lateral, superior.
• Segment III: Left lateral, inferior.
• Segment IVa & IVb: Left medial superior/inferior (close to gallbladder fossa).
• Segment V: Right anterior, inferior.
• Segment VI: Right posterior, inferior.
• Segment VII: Right posterior, superior.
• Segment VIII: Right anterior, superior.

👉 Clinical relevance: Segment I is often spared in cirrhosis, hypertrophying due to its separate venous drainage. Surgeons plan segmentectomies, lobectomies, or extended hepatectomies based on this anatomy.

🔬 Microscopic Anatomy

• Hepatic lobules: Hexagonal units centred around a central vein.
• Portal triads: At corners — contain branches of portal vein, hepatic artery, and bile duct.
• Sinusoids: Fenestrated capillaries allowing exchange between blood and hepatocytes.
• Kupffer cells: Resident macrophages → important in immunity and clearance of pathogens.
• Space of Disse: Site of lymph formation, lies between hepatocytes and sinusoids.

💉 Blood Supply

• Portal Vein (75%): Nutrient-rich, oxygen-poor blood from GI tract.
• Hepatic Artery (25%): Oxygen-rich blood from celiac axis.
• Venous outflow via 3 hepatic veins → inferior vena cava.

⚠️ Clinical note: The dual supply protects against infarction. Portal vein thrombosis → varices/portal hypertension; hepatic artery thrombosis (esp. post-transplant) → graft failure.

🟡 Biliary System

• Bile canaliculi between hepatocytes → intralobular ducts → right & left hepatic ducts.
• Right + Left hepatic ducts = common hepatic duct.
• Joins cystic duct = common bile duct → duodenum (via ampulla of Vater).

👉 Disorders: gallstones, cholangiocarcinoma, strictures can all obstruct this flow.

🌐 Lymphatic Drainage & Innervation

• Lymph: ~25–50% of thoracic duct lymph originates from the liver.
• Main drainage: hepatic & celiac lymph nodes.
• Innervation:
  • Parasympathetic (vagus) → metabolic regulation.
  • Sympathetic (coeliac plexus) → vascular tone, blood flow.
  • Sensory → capsule pain (Glisson’s capsule stretch = RUQ pain in hepatitis, CHF).

⚕️ Key Functions

• Metabolism: Glucose storage (glycogen), gluconeogenesis, lipid & protein metabolism.
• Detoxification: Drugs (CYP450), ammonia (→ urea cycle).
• Protein synthesis: Albumin, clotting factors, complement.
• Storage: Glycogen, iron, copper, fat-soluble vitamins.
• Bile production: Fat digestion, bilirubin excretion.

🧾 Clinical Relevance

• Cirrhosis: Architectural distortion → portal hypertension, varices, ascites.
• Hepatocellular carcinoma: Often arises in cirrhotic liver → resection guided by Couinaud segments.
• Liver transplantation: Segmental anatomy allows living-donor transplantation.
• Trauma: Liver is the most commonly injured abdominal organ → knowledge of segments guides haemostatic surgery.

✅ Conclusion

The liver’s anatomy spans from macroscopic lobes and Couinaud’s functional segments to microscopic lobules with portal triads and sinusoids. Its dual blood supply, biliary network, and segmental architecture underpin both physiology and surgery. A clear grasp of this anatomy is indispensable for managing liver disease, planning resections, and performing transplantation safely.