Acid–Base Balance

The regulation of the blood’s acidity, measured as pH and normally held within a very tight range around 7.4. Even small deviations disrupt the proteins and enzymes on which every cell depends.

Three lines of defense

• Buffers — chemicals in the blood that instantly soak up excess acid or base; the fastest response, acting in seconds.
• The lungs — exhaling carbon dioxide removes acid; breathing faster or slower adjusts pH within minutes.
• The kidneys — excrete acid and adjust buffer levels; the slowest response, acting over hours to days, but the most powerful.

Key idea

pH is defended by three mechanisms working on three timescales — chemical buffers, then the lungs, then the kidneys. Because carbon dioxide is acidic, breathing and acid–base balance are tightly linked; see also blood gas regulation.