O2-Hb dissociation curve is shifted to left by –
**Core Concept**
The oxygen-hemoglobin dissociation curve is a graphical representation of the relationship between the partial pressure of oxygen (pO2) and the saturation of hemoglobin with oxygen. A left shift in the curve indicates that hemoglobin has a higher affinity for oxygen, making it more difficult for oxygen to be released to the tissues.
**Why the Correct Answer is Right**
A left shift in the O2-Hb dissociation curve is caused by an increase in 2,3-bisphosphoglycerate (2,3-BPG) levels, which binds to hemoglobin and decreases its affinity for oxygen. This is also seen in conditions such as anemia, where the body produces more 2,3-BPG to compensate for the reduced oxygen-carrying capacity of the blood. Additionally, a left shift can be caused by decreased temperature, increased CO2 levels, and increased pH (alkalosis).
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because an increase in 2,3-BPG levels, which causes a left shift, is not typically seen in conditions such as chronic obstructive pulmonary disease (COPD).
**Option B:** This option is incorrect because a decrease in 2,3-BPG levels, which causes a right shift, is not typically seen in conditions such as anemia.
**Option C:** This option is incorrect because a decrease in temperature, which causes a left shift, is not typically seen in conditions such as hyperthyroidism.
**Clinical Pearl / High-Yield Fact**
In conditions where there is an increased demand for oxygen, such as during exercise, the body can increase 2,3-BPG levels to cause a left shift in the O2-Hb dissociation curve, allowing more oxygen to be released to the tissues.
**Correct Answer: D. Increased CO2 levels**