The control of extracellular acid-base balance is difficult to grasp unless it is realized that the pH of the blood depends upon the ratio H2CO3/NaHCO3 and that, without alteration of pulmonary ventilation, the H2CO3 neither rises nor falls. It must also be understood that the H+ ions, excreted by the kidneys, are byproducts of NaHCO3 reclamation and neogenesis and are not derived from metabolic acids.