pH
pH is a simple number that tells you how acidic or alkaline a fluid is. Lower numbers mean more acidic, higher numbers more alkaline, with 7 being neutral. The body keeps the pH of arterial blood within a remarkably narrow range, very close to 7.4.
This tight control matters because enzymes and proteins only work properly within a narrow acidity range; even small, sustained drifts impair the heart, brain and metabolism. The body defends pH with three layers: fast chemical buffers such as bicarbonate in the blood, the lungs adjusting carbon dioxide within minutes, and the kidneys adjusting acid and bicarbonate over hours to days.
Because carbon dioxide forms acid, breathing is a powerful and rapid pH lever: breathing faster removes acid and raises pH, breathing less retains acid and lowers it. This is why disorders of breathing and disorders of metabolism both show up as changes in blood pH, and why interpreting pH alongside carbon dioxide and bicarbonate is central to diagnosing acid-base problems.
A blood pH below about 7.35 is called acidemia and above about 7.45 is alkalemia; the underlying processes that push pH are termed acidosis and alkalosis.