The regulation of the concentrations of acids and bases in blood and other body fluids so that the pH remains within a physiologically acceptable range (see ph scale). This is achieved by the presence of natural buffer systems, such as the haemoglobin, hydrogencarbonate ions, and carbonic acid in mammalian blood. By acting in conjunction, these effectively mop up excess acids and bases and therefore prevent any large shifts in blood pH. The hydrogencarbonate buffer system is regulated by the kidneys, in which there is net excretion of hydrogen ions, while hydrogencarbonate ions are conserved, and by the lungs, which control the level of carbon dioxide in the blood.