Makindo Medical Notes

Related Subjects: |Anatomy of the Ear |Anatomy of the Oesophagus |Anatomy of the Diaphragm |Anatomy of Large Bowel |Anatomy of Small Bowel |Anatomy of the Biliary system |Anatomy of the Liver |Anatomy of the Eye |Anatomy of the Larynx |Anatomy of the Ear |Anatomy of Male Genitalia |Anatomy of Skin

Introduction

• The liver is the largest solid organ in the human body and plays a central role in metabolism, detoxification, protein synthesis, and bile production.
• Its complex architecture, ranging from its gross lobar anatomy to its detailed microscopic organization, underpins its many functions.
• A key aspect of liver anatomy—especially for surgical planning—is its division into distinct segments, each with its own vascular inflow, biliary drainage, and functional capacity.

Gross Anatomy

The liver is located in the right upper quadrant of the abdomen, beneath the diaphragm, and extends across the midline. Traditionally, it is divided into two main lobes:

• Right Lobe: The larger lobe, which is further subdivided into segments based on vascular and biliary anatomy.
• Left Lobe: The smaller lobe, extending into the left upper quadrant.

For surgical purposes, the liver is further divided into eight functional segments according to the Couinaud classification. These segments are defined by the distribution of the portal vein, hepatic artery, and bile ducts.

Surgical Segments (Couinaud Classification) divides the liver into eight segments:

• Segment I (Caudate Lobe): Located on the posterior surface of the liver, it has independent vascular inflow and outflow.
• Segment II: Part of the left lateral sector, positioned superiorly and laterally.
• Segment III: Also part of the left lateral sector, located inferiorly to Segment II.
• Segment IV: Divided into IVa and IVb, this segment forms the medial portion of the left lobe.
• Segment V: Located in the anterior portion of the right lobe, inferiorly.
• Segment VI: Positioned in the posterior part of the right lobe, inferiorly.
• Segment VII: Found in the superior-posterior area of the right lobe.
• Segment VIII: Located in the anterior-superior part of the right lobe.

Each segment is anatomically distinct, with its own vascular supply and biliary drainage, which is critical for planning resections and liver transplantations.

Microscopic Anatomy

The liver is organized into microscopic units called lobules. Each lobule is roughly hexagonal and is centered around a central vein. The key components include:

• Hepatocytes: Arranged in plates radiating from the central vein, these cells are responsible for most metabolic and synthetic functions.
• Sinusoids: Capillary channels between hepatocyte plates that facilitate the exchange of nutrients, oxygen, and waste products. Kupffer cells, the resident macrophages, line these sinusoids.
• Portal Triads: Located at the corners of each lobule, each triad consists of a branch of the portal vein, a branch of the hepatic artery, and a bile duct, ensuring the dual blood supply and bile transport.

Blood Supply

The liver receives blood from two main sources:

• Portal Vein: Supplies approximately 75% of the liver’s blood, rich in nutrients from the gastrointestinal tract.
• Hepatic Artery: Provides about 25% of the blood, delivering oxygen-rich blood from the systemic circulation.

Venous drainage is via the hepatic veins, which drain into the inferior vena cava.

The Biliary System

Hepatocytes produce bile, which is collected by bile canaliculi. These canaliculi merge to form progressively larger bile ducts, culminating in the right and left hepatic ducts. The hepatic ducts join to form the common hepatic duct, which in turn joins with the cystic duct from the gallbladder to form the common bile duct, directing bile to the duodenum.

Lymphatic Drainage and Innervation

• Lymphatic Drainage: The liver has an extensive lymphatic network that drains mainly into the hepatic and celiac lymph nodes, playing a role in immune surveillance.
• Innervation: Both sympathetic and parasympathetic fibers innervate the liver. The sympathetic fibers, derived from the thoracic spinal cord, and the parasympathetic fibers via the vagus nerve, help regulate hepatic blood flow and metabolic activity.

Functions and Clinical Relevance

The liver’s intricate structure underpins its diverse functions:

• Metabolic Regulation: It is central to carbohydrate, protein, and lipid metabolism.
• Detoxification: The liver metabolizes toxins and drugs, facilitating their excretion.
• Synthetic Function: It produces essential proteins (e.g., albumin, clotting factors) and bile for digestion.
• Storage: The liver stores glycogen, vitamins, and minerals, serving as a reservoir of energy and nutrients.

Detailed knowledge of liver anatomy, particularly its surgical segments, is essential for the planning of hepatic resections, transplantations, and other interventional procedures.

Conclusion

The liver's anatomy is characterized by its gross lobar divisions, microscopic lobular organization, and the division into eight functional surgical segments as defined by the Couinaud classification. This structural complexity enables its critical roles in metabolism, detoxification, synthesis, and storage. A comprehensive understanding of liver anatomy, including its surgical segmentation, is indispensable for clinicians managing hepatic diseases and planning liver surgeries, ultimately improving patient outcomes.