Water & nutrient transport
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