The compensatory shift of the oxyhemoglobin dissociation curve toward normal in chronic (>24 hours) respiratory alkalosis is due to what?

The compensatory shift of the oxyhemoglobin dissociation curve toward normal in chronic (>24 hours) respiratory alkalosis is due to what?

• A. Increased renal excretion of HCO3−
• B. An influx of potassium into red blood cells
• C. Altered erythrocyte 2,3-diphosphoglycerate metabolism
• D. Decreased sensitivity of the central nervous system to changes in PaCO2

Answer: (C)

Oxyhemoglobin affinity for oxygen is adjusted by red blood cell factors, and this curve can shift to help preserve tissue oxygen delivery during acid–base disturbances. In chronic respiratory alkalosis, the blood becomes more alkaline, which initially shifts the curve to the left (hemoglobin holds onto O2 more tightly and unloading to tissues is reduced). Over more than a day, the body not only reduces pH toward normal via renal excretion of bicarbonate, but red blood cells also alter 2,3-DPG metabolism. The change in 2,3-DPG levels modulates Hb–O2 affinity, producing a rightward shift that offsets the leftward effect of alkalemia and moves the curve back toward normal. This adjustment helps maintain adequate tissue oxygen delivery as pH settles. The other options don’t directly explain this curve reset: renal bicarbonate excretion addresses pH rather than Hb–O2 affinity; potassium shifts in red cells aren’t a known driver of the Hb dissociation curve; central nervous system changes in PaCO2 affect ventilation, not the Hb–O2 binding curve.

Oxyhemoglobin affinity for oxygen is adjusted by red blood cell factors, and this curve can shift to help preserve tissue oxygen delivery during acid–base disturbances. In chronic respiratory alkalosis, the blood becomes more alkaline, which initially shifts the curve to the left (hemoglobin holds onto O2 more tightly and unloading to tissues is reduced). Over more than a day, the body not only reduces pH toward normal via renal excretion of bicarbonate, but red blood cells also alter 2,3-DPG metabolism. The change in 2,3-DPG levels modulates Hb–O2 affinity, producing a rightward shift that offsets the leftward effect of alkalemia and moves the curve back toward normal. This adjustment helps maintain adequate tissue oxygen delivery as pH settles.

The other options don’t directly explain this curve reset: renal bicarbonate excretion addresses pH rather than Hb–O2 affinity; potassium shifts in red cells aren’t a known driver of the Hb dissociation curve; central nervous system changes in PaCO2 affect ventilation, not the Hb–O2 binding curve.