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How does blood help keep its pH near 7.4?
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blood
Blood sediment in test tubes, showing plasma (clear fluid), red blood cells (red), and the release of hemoglobin into the surrounding plasma (pink).
Blood maintains a stable pH of approximately 7.4 through a combination of chemical buffering systems and physiological regulation by the lungs and kidneys.12
Chemical Buffering
Blood contains natural buffer systems that resist changes in acidity by inactivating added hydrogen ions. The most significant of these are blood proteins, particularly hemoglobin, which effectively bind hydrogen ions. Additionally, the conversion of carbon dioxide into carbonic acid—facilitated by the enzyme carbonic anhydrase—results in the formation of bicarbonate ions, which serve as a critical buffer base.34
Physiological Regulation
- Respiratory Control: The lungs regulate blood pH by controlling the concentration of carbon dioxide, an acidic byproduct of metabolism. A sensing mechanism in the brain monitors carbon dioxide levels and adjusts the rate and depth of breathing to eliminate excess carbon dioxide, thereby preventing the blood from becoming too acidic.1
- Renal Excretion: The kidneys manage long-term acid-base balance by excreting nonvolatile acids in the urine and reabsorbing bicarbonate to maintain plasma concentrations.4