The pH of arterial blood is maintained near _____ at rest.

• A. 7.45
• B. 7.50
• C. 7.35
• D. 7.40

Answer: (D)

Arterial pH is kept within a very tight window because many bodily processes rely on it staying stable. At rest, it sits around 7.40, with the normal reference range about 7.35 to 7.45. This near-constant value is achieved by the interaction of buffering systems in the blood and the coordinated actions of the lungs and kidneys: the lungs adjust carbon dioxide levels to influence acidity, while the kidneys regulate bicarbonate and hydrogen ion excretion to compensate for metabolic acids produced by the body. Values around 7.35 or 7.45 are still normal at the edges of this range, but 7.50 would indicate a higher-than-normal alkalinity. So the resting arterial pH is approximately 7.40.