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The largest internal organ in the body. Its soft, spongy tissue spreads like a flattened football between the diaphragm and the stomach, tucked protectively beneath the lower ribs on the right side of the abdomen. Weighing about three and a half pounds, the liver contains 15 percent of the body’s blood (about a pint).
About 60 percent of this blood is venous and comes from the gastrointestinal tract and spleen, entering the liver via the portal vein. The venous blood delivers nutrients that the liver further metabolizes, the products of which it then sends back into the bloodstream. The hepatic artery, which branches directly from the abdominal aorta, delivers oxygenated blood to the liver to fuel the functions of its cells. On the underside of the liver is the gallbladder, which concentrates and stores the bile the liver produces.
The liver’s two main lobes, the small left lobe and the large right lobe, support an intricate network of lobules, thousands of tiny communities of hepatocytes (the cells that carry out the liver’s functions) that filter nutrients, wastes, bacteria, and toxins from the blood. The microscopic spaces between the lobules are the sinusoids, into which the blood from the portal vein drains. Each lobule is a hexagonal structure two layers of cells deep and several cells horizontally and vertically in a platelike configuration.
At the vertical junctions of the lobules are the portal triads, each containing three microscopic structures: a venule, an arteriole, and a bile duct. The portal triads collect the substances the lobules produce and convey them to the larger vessels that will carry them out to the structures of the body. The membranous connective tissue that envelopes the liver also extends like a web through the liver, providing a supportive structure for the lobules, sinusoids, and portal triads.
The lobules are the work stations of the liver. They metabolize nutrients and toxins, and synthesize (manufacture) numerous substances including amino acids, proteins essential for plasma production, lipoproteins, cholesterol, immune factors, clotting factors, lymph, and bile. the lobules convert glucose to glycogen, a storage form of glucose the body can draw from when blood levels of glucose fall, and glycogen back to glucose, processes that integrate closely with the balance of glucose and insulin in the blood.
The lobules also deconstruct old erythrocytes (red blood cells) to recycle the iron and bilirubin they contain. Specialized phagocytic (“cell eating”) cells, called Kupffer cells, reside in the sinusoids to consume bacteria and cellular waste. The liver stores iron, glycogen, vitamin A and vitamin B12, and other chemicals the body needs for cellular activities. The liver is unique among the body’s organs in its ability to regenerate itself. This extraordinary capacity speaks to the significant extent of damage that must take place to permanently destroy liver tissue. Even so, the liver can meet the needs of the body as long as 25 percent of its cells remain functional.
Liver Diseases and Disorders
For further discussion of the liver within the context of gastrointestinal structure and function, please see the overview section “The Gastrointestinal System.”
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