Ascites

Ascites is the accumulation of fluid (usually serous fluid which is a pale yellow and clear fluid) in the abdominal (peritoneal) cavity. The abdominal cavity is located below the chest cavity, separated from it by the diaphragm. Ascitic fluid can have many sources such as liver disease, cancers, congestive heart failure, or kidney failure.

The most common cause of ascites is advanced liver disease or cirrhosis. Approximately 80% of the ascites cases are thought to be due to cirrhosis. Although the exact mechanism of ascites development is not completely understood, most theories suggest portal hypertension (increased pressure in the liver blood flow) as the main contributor. The basic principle is similar to the formation of edema elsewhere in the body due to an imbalance of pressure between inside the circulation (high pressure system) and outside, in this case, the abdominal cavity (low pressure space). The increase in portal blood pressure and decrease in albumin (a protein that is carried in the blood) may be responsible in forming the pressure gradient and resulting in abdominal ascites.

Other factors that may contribute to ascites are salt and water retention. The circulating blood volume may be perceived low by the sensors in the kidneys as the formation of ascites may deplete some volume from the blood. This signals the kidneys to reabsorb more salt and water to compensate for the volume loss.

Some other causes of ascites related to increased pressure gradient are congestive heart failure and advanced kidney failure due to generalized retention of fluid in the body.

In rare cases, increased pressure in the portal system can be caused by internal or external obstruction of the portal vessel, resulting in portal hypertension without cirrhosis. Examples of this can be a mass (or tumor) pressing on the portal vessels from inside the abdominal cavity or blood clot formation in the portal vessel obstructing the normal flow and increasing the pressure in the vessel (for example, the Budd-Chiari syndrome).

There is also ascites formation as a result of cancers, called malignant ascites. These types of ascites are typically manifestations of advanced cancers of the organs in the abdominal cavity, such as, colon cancer, pancreatic cancer, stomach cancer, breast cancer, lymphoma, lung cancer, or ovarian cancer.

Pancreatic ascites can be seen in people with chronic (long standing) pancreatitis or inflammation of pancreas. The most common cause of chronic pancreatitis is prolonged alcohol abuse. Pancreatic ascites can also be caused by acute pancreatitis as well as trauma to the pancreas.

Source: MedicineNet.com
http://www.medicinenet.com/ascites/article.htm