The pH scale measures the concentration of hydrogen ions in a solution. The numbers on the scale refer to the exponent of the concentration of hydrogen ions. In a solution with a pH of 3, one in every 10³ molecules is a hydrogen ion. In a solution with a pH of 4, one in 10⁴ molecules is a hydrogen ion, 10 times less. Fluids in living things must be kept within a narrow range of pH for proper functioning of the enzymes involved in biochemical reactions. Buffering systems are used to maintain pH.