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Total Body Fluid Composition (Fig. 17-1). The adult body is composed of approximately 60% water, with some variation with age and gender, as well as significant variation. The cell membrane prevents sodium, the primary extracellular cation, from moving into the cell, except for a small amount by active pump transport, but isotonic fluids containing sodium added to the vascular space are distributed throughout the extracellular volume so that only 20% of the volume infused remains in the plasma.