An increase in which of the following parameters will shift the O2 dissociation curve to the left?
**Core Concept**
The O2 dissociation curve represents the relationship between the partial pressure of oxygen (pO2) and the saturation of hemoglobin with oxygen (SaO2). The curve's position and shape are influenced by various factors that affect the affinity of hemoglobin for oxygen. A left shift in the O2 dissociation curve indicates an increased affinity of hemoglobin for oxygen, making it easier to bind and release oxygen.
**Why the Correct Answer is Right**
An increase in 2,3-bisphosphoglycerate (2,3-BPG) levels will shift the O2 dissociation curve to the left. This is because 2,3-BPG binds to deoxyhemoglobin, increasing its affinity for oxygen. As a result, hemoglobin binds oxygen more readily, and the curve shifts to the left. This mechanism is particularly important in conditions such as anemia, where increased 2,3-BPG levels help compensate for the reduced oxygen-carrying capacity of the blood.
**Why Each Wrong Option is Incorrect**
**Option A:** An increase in pH (alkalosis) will shift the O2 dissociation curve to the right, not the left. This is because alkalosis decreases the affinity of hemoglobin for oxygen.
**Option B:** An increase in temperature will also shift the O2 dissociation curve to the right, as higher temperatures increase the kinetic energy of hemoglobin and reduce its affinity for oxygen.
**Option C:** An increase in CO2 tension (hypercapnia) will shift the O2 dissociation curve to the right, as CO2 binds to hemoglobin and reduces its affinity for oxygen.
**Clinical Pearl / High-Yield Fact**
Remember that 2,3-BPG levels are increased in anemia, providing a compensatory mechanism to improve oxygen delivery to tissues. This is a critical concept in understanding the pathophysiology of anemia and the body's adaptive responses to reduced oxygen-carrying capacity.
**Correct Answer:** C.