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.
Simply put, the body swells naturally from prolonged exposure to salt water – resulting in their bodies appearing bloated when they exit the water. This rings especially true when the swimmers are in the water for over two hours – which is just a bit longer than the world’s best open water swimmers take to complete 10 kilometers.
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.
I can already see the new excuse forming, “I’m not overweight, I’m just third-spacing today.”