Metabolic and biochemical reactions
Water molecules are consumed during the digestive process as they break down complex foods into units that are easier to absorb. Inversely, certain metabolic reactions create water in our bodies.
We have approximately 5 liters of blood in our body. Our two kidneys filter about 180 liters of blood volume a day (125 ml/minute), performing this filtration some 50 to 60 times a day(2).
Yet we only urinate 1.2 to 2 liters of urine daily. The kidneys have extraordinary powers of filtration and concentration!
Sources: (1) Haüssinger D., The role of cellular hydration in the regulation of cell function, Biochemical Journal Feb 01, 1996, 313 (3) 697-710;
; (2) Valtin and Schafer, Renal function: mechanisms preserving fluid and solute balance in health 3rd ed.1995