Third spacing (noun): In human physiology, extracellular fluids are distributed between the interstitial compartment (i.e. tissue) and intravascular compartment (i.e. plasma) in an approximately 75%-25% ratio.
Third spacing is the physiological concept that body fluids may collect in a "third" body compartment that isn't normally perfused with fluids. For example, open water swimmer’s body to appear waterlogged or swollen after a long swim when fluid is trapped in the interstitial spaces in the brain, lungs, abdomen and extremities.
Third spacing can be caused by a loss of electrolytes. In turn, this results in extracellular fluids going out of the blood vessels and into the skin tissue that normally is not perfused with fluids. The marathon swimmer looked soft and pudgy when he exited the water after his crossing of the English Channel.
Excerpt from the Open Water Swimming Dictionary (English version 2009). Photo of the third spacing effect on Grant Hackett at the 2008 World Open Water Swimming Championships.
Copyright © 2009 by World Open Water Swimming Association