Open Water Swimming Word Of The Day: Third Spacing Courtesy of WOWSA, Huntington Beach, California. 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 […]