Living organisms depend on a proper balance of hydrogen and hydroxide ions in order to maintain essential physiological processes. Scientists use pH to express the concentration of hydrogen ions in a solution. Many organisms have tightly regulated systems to maintain the pH within the required range.
The normal pH of human blood is slightly basic at 7.4. Illness and environmental factors affect normal blood pH. If the individual is having difficulty breathing, carbon dioxide builds up in the blood leading to acidosis. The dissolved carbon dioxide lowers the pH of the blood. Untreated, the condition becomes dangerous to the individual. On the other hand, an increase in bicarbonate or low levels of carbon dioxide in the blood leads to alkalosis. The change in blood pH causes muscle twitches and fatigue. Untreated alkalosis sometimes progresses to paralysis or death.
The pH affects the function of enzymes in organisms, including humans. These chemicals, necessary for proper function of life processes, lose their effectiveness outside the proper pH range. Complex organisms such as mammals have areas within the body that function best at vastly different pH levels. Human saliva has a pH of 7.0, but gastric juices have an extremely acidic pH of 2.0. While antacids neutralize heartburn, they make the stomach juices close to neutral and slow digestion.