The organs of the abdomen are contained in a sac or membrane called the peritoneum. Normally the peritoneal cavity contains no fluid, although in women a small amount (20ml, or less than an ounce) can occasionally be found depending on the menstrual cycle. Ascites is the term used to denote a fluid collection in the peritoneal cavity, a situation that is not normal.

There are a variety of diseases that can cause the fluid to accumulate and the reasons that the ascites occurs may be different for each disease. Cancer that spreads to the peritoneum can cause direct leakage of fluid, while other illnesses cause an excess accumulation of water and sodium in the body. This fluid can eventually leak into the peritoneal cavity.

Most commonly, ascites is due to liver disease and the inability of that organ to produce enough protein to retain fluid in the bloodstream. Normally, water is held in the bloodstream by oncotic pressure. The pull of proteins keeps water molecules from leaking out of the capillary blood vessels into surrounding tissues. As liver disease advances, its ability to manufacture proteins is decreased, so oncotic pressure decreases because of lack of total protein in the body, and water leaks into surrounding tissues.

In addition to ascites, the extra fluid can be appreciated in many other areas of the body as edema (swelling). Edema can occur in the feet, legs, chest cavity, a variety of other organs, and fluid can accumulate in the lungs. Symptoms caused by this excess fluid will depend upon its location.

The list of causes of ascites begins with the liver. Regardless of the reason for liver failure, a malfunctioning liver cannot make enough protein to maintain oncotic pressure to keep fluid in the circulatory system.

Causes of ascites due to liver problems include:
-- Cirrhosis describes a form of liver failure in which liver tissue that is damaged is replaced by scar tissue. As more liver tissue is lost is progressive liver failure occurs. Alcoholic liver disease or alcoholic hepatitis (hepar=liver +itis=inflammation), Viral Hepatitis (B or C), and fatty liver disease are the most common causes for Cirrhosis.

-- Acute liver failure can result in ascites. This may be due to any acute injury to liver cells including adverse reactions to medications or drug abuse (for example, liver failure is the major consequence of acetaminophen overdose).

-- Budd-Chiari syndrome is caused by the blockage of the hepatic veins (those that drain the liver). This causes the triad of ascites, abdominal pain, and hepatosplenomegaly (enlargement of the liver and spleen).

-- Cancer that has metastasized or spread to the liver can also be the source of ascites.