Introduction

The P50 value of hemoglobin (Hb) represents the partial pressure of oxygen (pO2) at which hemoglobin is 50% saturated. During exercise, the P50 value increases, indicating decreased affinity of hemoglobin for oxygen, facilitating oxygen unloading to active tissues. Multiple physiological changes contribute to this, including alterations in CO2 concentration, pH, temperature, and 2,3-bisphosphoglycerate (2,3-BPG) levels. This article reviews the mechanisms behind P50 changes and uncovers which common statements are incorrect.

Analysis of the Proposed Statements

A. Increased CO2 production by muscles elevates pCO2 of blood which affects P50 value.

• True.
  Elevated CO2 concentration in blood during exercise leads to the Bohr effect, decreasing Hb’s oxygen affinity and increasing P50, facilitating oxygen delivery.

B. The affinity of hemoglobin for oxygen increases as 2,3-bisphosphoglycerate (2,3-BPG) level is elevated.

• False.
  Increased 2,3-BPG decreases hemoglobin’s oxygen affinity, causing a rightward shift in the dissociation curve and increasing P50, which promotes oxygen unloading, not increased affinity.

C. Increased body temperature shifts the oxyhemoglobin dissociation curve to the left.

• False.
  Increased temperature during exercise causes a rightward shift of the dissociation curve, reducing Hb affinity for oxygen, raising P50.

D. The decreased pH of blood reduces the affinity of hemoglobin for oxygen.

• True.
  Lower pH (acidosis), as found in exercising muscle tissues, reduces hemoglobin’s oxygen affinity through the Bohr effect, increasing the P50 value.

Summary Table

Correct Answer Choice for Incorrect Statements

The incorrect statements are:

(2) B and C

Physiological Context

• During exercise, muscles generate CO2 and heat, and produce lactic acid, lowering pH. These factors act together to shift the oxygen dissociation curve rightward, increasing P50 and enabling more oxygen release to tissues.

• Additionally, 2,3-BPG, a glycolytic intermediate in red blood cells, accumulates to further facilitate oxygen unloading.

Conclusion

Among the provided statements, the incorrect ones describing the mechanism of increased P50 during exercise are:

(2) B and C