The liver is one of the most important organs in the body, playing many roles, including filtering harmful substances from the blood, digesting fats from food and storing the sugar that your body uses for energy. It lies on the right side of the abdominal cavity beneath the diaphragm. It is glossy in appearance and dark red due to the rich supply of blood flowing through it. It is the largest solid organ in the body, weighing about three and a half pounds, and measuring about 8 inches across. It also the most resilient of all the body's organs, and even capable of regenerating itself. When part of the liver is removed, it will often grow back to its original size. As little as 25% of a liver can regenerate into a whole liver.
When viewed under a microscope, the liver is seen as a large network of units called hepatic lobules. The hepatic lobule is very small and looks like a six-sided cylinder. The actual lobule is surrounded by connective tissue and has about 6 clusters of vessels around the edges. These vessels are branches of the portal vein, hepatic artery, and the bile duct. Through the middle of the lobe is a central vein that is surrounded by cords of liver cells that expand outward in all directions. There are also wide thin-walled blood vessels called sinusoids mixed in with the cords of liver cells. 
The anterior surface of the liver is triangular in shape, and made up of two lobes. The right lobe is the larger of the two, measuring 6 to 7 inches in length. The left lobe is about 3 inches in length.The falciform ligament is visible on the anterior side of the liver, dividing it into a left anatomical lobe, and a right anatomical lobe. There are also two more lobes. With the liver flipped over, the two lobes are located between the two bigger lobes. They are the caudate lobe and the quadrate lobe.
The liver has multiple important functions in the body. In fact, most of the functions are all interlaced. It changes food into energy, cleans poisons from the blood whether ingested or inhaled, makes bile to help in the digestion of food, produces substances that break-down fats, converts glucose to glycogen, and converts ammonia into urea, which is the main substance of urine.
The process of converting ammonia from the body into urea is such an important function of the liver. In the presence of severe liver disease, ammonia accumulates in the blood because of decreased blood clearance and a malfunction in converting it properly into urea. Too much ammonia in the body can be toxic, especially to the brain.
The liver also makes amino acids and stores vitamins and minerals (vitamins A, D, K and B12). In addition to the seemingly endless number of important functions the liver has, it is also responsible for producing 80% of all the cholesterol in the body. By regulating blood levels of substances such as cholesterol and glucose, the liver helps maintain body homeostasis. 
The liver also plays an important role in hormonal changes and inactivation of hormones. Chronic liver disease may cause hormonal imbalances. For example, the masculinizing hormone testosterone and the feminizing hormone estrogen are metabolized and inactivated by the liver. For someone with liver damage, especially due to alcohol abuse, the result for a man could be the hyperactivity of estrogen which gives them more female characteristics.
The circulatory system of the liver is unlike that of any other organ. It is important to know that majority of the liver's blood supply is venous blood. That means that the blood comes from veins. About three-fourths of all the blood that enters the liver is blood from the hepatic portal vein. The remaining one-forth of the blood supply to the liver is from the hepatic artery. The hepatic artery carries oxygen-rich blood from the aorta to the liver.
The blood vessels in the liver subdivide repeatedly until they are tiny capillaries. Each capillary leads to one of thousands of lobules that make up the tissues of the liver. The lobules are made up of hepatic cells, which are the basic metabolic cells of the liver. 
All of the venous blood returning from the small intestine, stomach, pancreas, and spleen enters into the portal vein and reaches the liver. This means that the liver gets exposed to basically everything that has been absorbed through those small intestine, where practically all nutrients in the body are taken from. This explains how colon cancer cells can leave the intestine and travel through the portal vein, into the liver, and then grow into tumors. 
- Main Article: Gallbladder
Attached to the side of the liver is a small sac-like structure called the gallbladder. The gallbladder is an important structure in the biliary system. During periods of time when bile is not flowing into the intestine, the excess bile gets dehydrated and stored in the gallbladder.  The major components of bile include cholesterol, phospholipids, bilirubin (a metabolite of red blood cell hemoglobin), and bile salts. Bile salts act as "detergents" that help digest and absorb dietary fats. Damage to the liver or an obstruction of some kind in the bile duct (called a gallstone) can lead to serious problems such as cholestasis, steatorrhea, or jaundice.
Although the liver is very resilient it is can become diseased. There are many kinds of liver diseases, such as those caused from viruses like hepatitis A, hepatitis B, and hepatitis C. Those can all be prevented. 
Excessive alcohol consumption over a long period of time is the main cause of liver disease. Too much alcohol can make the liver swell with fat. This is known as "fatty liver". If the fat becomes inflamed, it can lead to either alcoholic hepatitis or cirrhosis. Alcoholic hepatitis is a liver problem that causes serious damage to the liver but can be healed. Cirrhosis is much more serious. It causes irreversible liver damage. Cirrhosis causes easy bruising or bleeding, swelling of the abdomen or legs, hypersensitivity to medications, high blood pressure in the vein entering the liver, enlarged veins in the esophagus and stomach, and kidney failure. Because of extensive scarring, a liver with cirrhosis shrinks considerably compared to a normal liver.
Another common disease of the liver is cancer. There are two types of liver cancer that are classified. Primary liver cancer starts in the liver. Metastatic liver cancer starts somewhere else in the body and eventually spreads to your liver. Symptoms of liver cancer include a lump or pain on the right side of your abdomen and yellowing of the skin (called jaundice). However, sometimes liver cancer is hard to detect until it is already in advanced stages of development. This makes it harder to treat. Treatment options include surgery, radiation, chemotherapy or liver transplantation. 
- Your Liver FunctionsJanis and Friends Hepatitis C Website
- Liver Mama's Health
- Architecture of the Liver and Biliary Tract R. Bowen, June 5, 2004.
- Hepatitis MedlinePlus
- Cirrhosis Medline Plus
- Liver Cancer Medline Plus