The 180 l of glomerular filtrate formed each day contain some 1100 g (2.5 pounds) of sodium chloride, of which only 5–10 g are excreted in the urine – 95% is reabsorbed by the tubules. Some 425 g (nearly a pound) of sodium bicarbonate and 145 g of glucose are filtered, and more than 99% of both are reabsorbed. Also filtered, only to be reabsorbed, are substantial quantities of potassium, calcium, magnesium, phosphate, sulfate, amino acids, vitamins, and many other substances valuable to the body.
It is no exaggeration to say that the composition of the blood is determined not by what the mouth takes in but by what the kidneys keep: they are the master chemists of our internal environment, which, so to speak, they manufacture in reverse by working it over completely some fifteen times a day…Our bones, muscles, glands, even our brains are called upon to do only one kind of physiological work, but our kidneys are called upon to perform an innumerable variety of operations. Bones can break, muscles can atrophy, glands can loaf, even the brain can go to sleep, without immediately endangering our survival; but should the kidneys fail to manufacture the proper kind of blood neither bone, muscle, gland nor brain could carry on (1).