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The adult male body is about 60% water for a total water content of some 42 litres (9. 2 imp gal; 11 US gal). This is made up of about 19 litres (4. 2 imp gal; 5. 0 US gal) of extracellular fluid including about 3. 2 litres (0. 70 imp gal; 0. 85 US gal) of blood plasma and about 8. 4 litres (1. 8 imp gal; 2. 2 US gal) of interstitial fluid, and about 23 litres (5. 1 imp gal; 6. 1 US gal) of fluid inside cells. The content, acidity and composition of the water inside and outside cells is carefully maintained. The main electrolytes in body water outside cells are sodium and chloride, whereas within cells it is potassium and other phosphates.

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