The liver and biliary system are vital parts of the digestive system. All nutrients and toxins that enter the body are eventually processed through the liver. The liver produces and regulates bile, a greenish-yellow fluid necessary for digestion, and breaks down substances into products that the body can either use or eliminate.

The liver is located mainly on the upper right side of the abdomen just under the rib cage. It is about the size of a football and weighs 3 to 4 pounds (1.3 to 1.8 kilograms). Healthy livers are cone-shaped with a smooth, rubbery texture and a reddish-brown color. The liver has two large parts, called lobes. Each lobe is made up of many small units called lobules, which contain tiny blood vessels.

The unusual role of the liver is characterized by its unique blood supply. Most organs receive oxygenated blood through one major artery and pass deoxygenated blood to the heart through one main vein. The liver receives oxygenated blood through the hepatic artery. In addition, the liver receives deoxygenated blood through the hepatic portal vein. This deoxygenated blood from the intestines contains all the nutrients and toxins from digested food that the liver must process. These vessels direct blood through the lobules, so the body’s entire blood supply eventually passes through the liver. The blood leaves the liver through the hepatic vein and returns to the heart.

Bile, which is produced and regulated by the liver, is essential to digestion. It consists of cholesterol, bile salts (also called bile acids) and bilirubin,  a substance formed from the breakdown of red blood cells that gives bile its color. It is transported through the biliary system, which includes the gallbladder (a small, pear-shaped organ located underneath the liver) and the bile ducts located inside and outside the liver. Bile ducts are tubes that carry bile between organs.

Bile leaves the liver through the hepatic duct and travels to and from the gallbladder through the cystic duct. The junction of the cystic and hepatic ducts forms the common bile duct, which transports bile to the portion of the small intestine called the duodenum.

Some bile flows directly from the liver into the duodenum. However, about half of it is stored in the gallbladder. When food enters the duodenum after a meal, nerve signals are sent to the gallbladder causing it to contract and release more bile into the duodenum. In between meals, only a small amount of bile flows into the duodenum.

Although the gallbladder plays an important role in digestion, it is not vital for survival. Some patients have their gallbladders surgically removed to treat illness. After the gallbladder is removed, bile flows from the liver through the hepatic ducts, into the common bile duct and directly into the small intestine, instead of being stored in the gallbladder.

Bile has many functions. It assists in the digestion and absorption of fats. Bile also helps the body eliminate certain waste products, such as hemoglobin from red blood cells and excess cholesterol. It also helps the body absorb cholesterol, fats and fat-soluble vitamins by increasing the ability of these substances to be dissolved in liquids. Bile also stimulates the secretion of water by the large intestine to help move contents through the digestive system.

The liver also performs many other functions that pertain to digestion. These include:

• Storing vitamins, sugars, fats and other nutrients from food.
  
  
• Storing energy by stockpiling glycogen (carbohydrate) and fat.
  
  
• Processing and synthesis of protein, glucose and fats (including cholesterol).
  
  
• Breaking down toxic substances, such as alcohol and poisonous chemicals.
  
  
• Removing waste products from the body. Cells called Kupffer's cells line parts of the liver and remove wastes such as bacteria from the blood.

The liver also performs many functions not related to digestion, such as resisting infection by producing immune factors and removing bacteria from the bloodstream.

The body cannot function without the liver. However, the liver’s structure enables it to process many toxins and continue to function even after suffering great damage. This is because, unlike most other organs in the body, the liver has some ability to heal itself by repairing or replacing damaged cells. Scientists do not completely understand the liver’s ability to regenerate following injury, but new research indicates that a bile imbalance may signal the organ to regrow.

Constant damage to the liver over a period of years or sudden poisoning, however, can kill enough tissue to result in liver failure, which requires liver transplantation.

Researchers are currently studying ways to reduce the need for these surgeries and to predict the outcome of living-donor liver transplants when they are necessary.