Normal blood pH is 7.35-7.45. If normal compensatory mechanisms are deranged or one or more strong forms of acidosis or alkalosis is present, the pH may leave this range. Acidemia occurs when pH goes below 7.35; Alkalemia occurs when pH rises above 7.45.

Acidemia is both more common and better tolerated than alkalemia. A pH of 7.2, while grossly abnormal, may well be found in a sick patient who has few effects from that pH itself (only from the renal failure or other cause of the low pH. In contrast, a pH of 7.5 is quite dangerous.

The body's enzymes are designed to operate most effectively in a certain pH range. Most are efficient at a pH around 7.4, which is also the pH at which physiologic buffers operate properly. However, most enzymes are also capable of much better (but reduced) function at low pH than at high pH.

Fortunately, the body is also far more likely to encounter acidemia than alkalemia. While dehydration, excessive exercise, a variety of toxins, and respiratory failure are all common causes of acidosis, the causes of alkalosis generally include only overbreathing and a much more limited number of toxins.

Overbreathing is actually very difficult, both because it tends to produce alkalosis and because ventilatory drive decreases rapidly when low blood carbon dioxide is present. The reader who attempts to breathe rapidly and deeply for any prolonged period will soon find themself growing light headed - far before alkalemia can be reached. In contrast, the reader who attempts to exercise vigorously may at least come close to mild acidemia - which would be harmless in a fit individual.

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