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.
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