The body consists of a lot of water which is divided into three liquid compartments: the plasma, the interstitial compartment and the intracellular space. Body fluids contain a number of dissolved substances including sugars, amino acids, fat, and not least a variety of salts (ions). Among these are sodium, potassium, calcium and H+ ions (protons), the latter determines the pH in the body. The different salts are unevenly distributed in the various fluid compartments, and this distribution is physiologically important to maintain normal cell function in the body’s various tissues. Both water and salt levels, and body content of H+ ions, are therefore regulated very carefully. The kidneys play an active role in this regulation by excreting excess salt and water as needed. In addition, the kidneys excrete various wastes including urea, which is formed during the degradation of proteins.