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In brief: Water. Free fluid is typically water. The lining of your abdominal contents are slippery (your bowel, the lining of your organs) and it's thus wet inside your abdomen. It's normal to have a little free fluid in one's pelvis, as that's where fluid tends to pool.
In Netter's Atlas of Human Physiology, body water is broken down into the following compartments: 1  Intracellular fluid (2/3 of body water). 2  Extracellular fluid (1/3 of body water). 3  Plasma (1/5 of extracellular fluid). 4  Interstitial fluid (4/5 of extracellular fluid). 5  Transcellular fluid (a.k.a. third space, normally ignored in calculations). 6  Contai