Structure of the Liver

The liver consists of four sections, or lobes. There are two main lobes--the right lobe, which is by far the larger, and the left lobe. Two small lobes lie behind the right lobe.

Each lobe is made up of multisided units called lobules. Most livers have between 50,000 and 100,000 lobules. Each lobule consists of a central vein surrounded by tiny liver cells grouped in sheets or bundles. These cells perform the work of the liver. Cavities known as sinusoids separate the groups of cells within a lobule. The sinusoids give the liver a spongy texture and enable it to hold large amounts of blood.

The liver has an unusual blood supply system. Like other organs, the liver receives blood containing oxygen from the heart. This blood enters the liver through the hepatic artery. The liver also receives blood filled with nutrients, or digested food particles, from the small intestine. This blood enters the liver through the portal vein. In the liver, the hepatic artery and the portal vein branch into a network of tiny blood vessels that empty into the sinusoids.

The liver cells absorb nutrients and oxygen from the blood as it flows through the sinusoids. They also filter out wastes and poisons. At the same time, they secrete sugar, vitamins, minerals, and other substances into the blood. The sinusoids drain into the central veins, which join to form the hepatic vein. Blood leaves the liver through the hepatic vein.

Each lobule also contains bile capillaries, tiny tubes that carry the bile secreted by the liver cells. The bile capillaries join to form bile ducts, which carry bile out of the liver. Soon after leaving the liver, the bile ducts join together, forming the hepatic duct. The liver manufactures bile continuously, even if the small intestine is not digesting food. Excess bile flows into the gall bladder, where it is stored for later use. Bile from the liver and gall bladder flows into the small intestine through the common bile duct.