By: Joan G. Creager. Human Anatomy and Physiology 2nd Edition

The liver has numerous functional units called lobules. Each cylindrical lobule is about 2 mm in diameter and is completely surrounded by connective tissue. From the central vein in the center of a lobule, cords of the liver cells radiate out in all directions. In the spaces between these cords of cells are sinusoids, wide thin-walled blood vessels lined with endothelial cells and Kupffer cells. Around the periphery of each lobule are five to seven portal triads, a clusters of three vessels – a branch of the hepatic portal vein, a branch of the hepatic artery, and a bile duct.

Its main lobe and left lobes are separated by the falciform ligament, a membrane that is continuous with the peritoneum. Seen from the inferior side, the right lobe of the liver has two smaller lobes, the posterior caudate lobe and the inferior quadrate lobe. Also visible on the inferior surface are the gallbladder, common bile duct, hepatic portal vein, hepatic artery, and the inferior vena cava.

The stroma of the liver is neatly packed with a basic architecture of polygonal columns, called liver lobules. In some animals (see figure above), but not humans, the connective tissue separating lobules is thick enough that they are easily distinguished by light microscopy. Portal spaces at the corners between lobules have more connective tissue surrounding portal triads composed of a portal venule bundled with a hepatic arteriole, and a bile duct. Small lymphatics and autonomic nerves also run through this space.

The human liver has three to six portal triads per liver lobule. The triads define the corners of the polygonal lobules, with the centers defined by a central vein. Radiating from the center to the lobule periphery are branching, anastamosing plates of hepatocytes, one or two cells thick, separated by capillaries, the liver sinusoids. This gives a spongelike architecture to the lobule. The sinusoids arise from terminal branches of the portal veins at the lobule periphery and terminate in the central vein. As the central vein progresses along the length of a lobule, it receives the inflow from more and more sinusoids and increases in diameter until joining a larger sublobular vein at the lobule base.