How does the bicarbonate buffer system maintain systemic pH?

How does the bicarbonate buffer system maintain systemic pH?

• A. CO2 dissolves in water to form carbonic acid, equilibrates to bicarbonate and H+, maintaining blood pH near 7.4; respiratory and renal systems regulate CO2 and bicarbonate.
• B. Bicarbonate acts as a strong base dominating pH to 7.0.
• C. Buffer system uses phosphate as primary buffer at pH 2.
• D. HCl is produced to keep pH stable.

Answer: (A)

The main idea is that blood pH is held steady by the bicarbonate buffer pair, which links carbon dioxide metabolism to proton buffering and is finely tuned by the lungs and kidneys. In the plasma, carbon dioxide from tissues combines with water to form carbonic acid, which quickly dissociates into bicarbonate and a proton. Bicarbonate acts as a base by neutralizing added H+, while the proton is buffered by the carbonic acid part of the system. This balance keeps pH near 7.4. The lungs regulate the amount of CO2, effectively controlling the driving force for the carbonic acid/bicarbonate pair. When acidity rises, more CO2 is expelled, shifting the equilibrium toward CO2 and H2O and removing H+ in the process, which buffers the drop in pH. When alkalinity rises, CO2 is retained to push the reaction toward producing more H+ and HCO3-, counteracting the pH rise. The kidneys contribute by reabsorbing or generating bicarbonate and by secreting hydrogen ions, adjusting the [HCO3−] to maintain the proper ratio with CO2. This balance is often summarized by the Henderson–Hasselbalch relationship for the bicarbonate buffering system, pH ≈ 6.1 + log([HCO3−]/[CO2]), with CO2 levels reflecting respiratory control and bicarbonate levels reflecting renal control. Together, these mechanisms keep systemic pH around 7.35–7.45. Other options misstate the system: bicarbonate is not a strong base that fixes pH at 7.0, the phosphate buffer is not the primary extracellular buffer at physiological pH, and HCl production is not how systemic pH is stabilized.

The main idea is that blood pH is held steady by the bicarbonate buffer pair, which links carbon dioxide metabolism to proton buffering and is finely tuned by the lungs and kidneys. In the plasma, carbon dioxide from tissues combines with water to form carbonic acid, which quickly dissociates into bicarbonate and a proton. Bicarbonate acts as a base by neutralizing added H+, while the proton is buffered by the carbonic acid part of the system. This balance keeps pH near 7.4.

The lungs regulate the amount of CO2, effectively controlling the driving force for the carbonic acid/bicarbonate pair. When acidity rises, more CO2 is expelled, shifting the equilibrium toward CO2 and H2O and removing H+ in the process, which buffers the drop in pH. When alkalinity rises, CO2 is retained to push the reaction toward producing more H+ and HCO3-, counteracting the pH rise. The kidneys contribute by reabsorbing or generating bicarbonate and by secreting hydrogen ions, adjusting the [HCO3−] to maintain the proper ratio with CO2.

This balance is often summarized by the Henderson–Hasselbalch relationship for the bicarbonate buffering system, pH ≈ 6.1 + log([HCO3−]/[CO2]), with CO2 levels reflecting respiratory control and bicarbonate levels reflecting renal control. Together, these mechanisms keep systemic pH around 7.35–7.45.

Other options misstate the system: bicarbonate is not a strong base that fixes pH at 7.0, the phosphate buffer is not the primary extracellular buffer at physiological pH, and HCl production is not how systemic pH is stabilized.