Question

Which of the following statements is correct about the oxygen ($\text{O}_2$) dissociation curves (A and C) relative to curve B?

• A. Curve A represents favourable $\text{O}_2$ association with haemoglobin at low $[\text{H}^+]$.
• B. Curve C represents favourable $\text{O}_2$ association with haemoglobin at low $\text{pCO}_2$.
• C. Curve A represents favourable $\text{O}_2$ association with haemoglobin at low pH.
• D. Curve C represents favourable $\text{O}_2$ association with haemoglobin at high $\text{pO}_2$.

Hint:

Use the acronym "CADET, face Right!" to remember the factors that shift the oxygen-hemoglobin curve to the Right:
- C $\to$ $\text{CO}_2$ increase
- A $\to$ Acid ($[\text{H}^+]$ increase pH decrease)
- D $\to$ 2,3-DPG increase
- E $\to$ Exercise
- T $\to$ Temperature increase
A decrease in these factors shifts the curve to the Left.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
We are given a graph showing three oxygen dissociation curves:
- Curve B: The standard/normal physiological curve.
- Curve A: Shifted to the left of B.
- Curve C: Shifted to the right of B.
We need to determine which statement correctly describes the physiological conditions associated with these shifts.

Step 2: Key Formula or Approach:
The affinity of hemoglobin for oxygen is influenced by several factors:
1. Left Shift (Curve A): Indicates increased oxygen affinity (favourable $\text{O}_2$ association/loading). This occurs under conditions found in the lungs:
- Low partial pressure of carbon dioxide ($\text{pCO}_2$)
- Low hydrogen ion concentration ($[\text{H}^+]$), which means high pH
- Low temperature
- Low 2,3-BPG concentration
2. Right Shift (Curve C): Indicates decreased oxygen affinity (favourable $\text{O}_2$ dissociation/unloading to tissues). This occurs under active metabolizing tissue conditions:
- High $\text{pCO}_2$
- High $[\text{H}^+]$ (low pH)
- High temperature
- High 2,3-BPG concentration

Step 3: Detailed Explanation:
Let us evaluate the options:
- Option (A): "Curve A represents favourable $\text{O}_2$ association with haemoglobin at low $[\text{H}^+]$."
- Since Curve A is left-shifted, it represents higher affinity, which means more favourable association.
- Low $[\text{H}^+]$ (alkaline conditions) is indeed one of the primary drivers of a left shift. Therefore, this statement is biochemically and physiologically correct.
- Option (B): "Curve C represents favourable $\text{O}_2$ association with haemoglobin at low $\text{pCO}_2$."
- This is incorrect because Curve C is right-shifted (favours dissociation, not association). Also, low $\text{pCO}_2$ causes a left shift.
- Option (C): "Curve A represents favourable $\text{O}_2$ association with haemoglobin at low pH."
- This is incorrect because low pH (acidic) corresponds to high $[\text{H}^+]$, which shifts the curve to the right (Curve C), not left.
- Option (D): "Curve C represents favourable $\text{O}_2$ association with haemoglobin at high $\text{pO}_2$."
- This is incorrect because high $\text{pO}_2$ is associated with normal loading, but Curve C is specifically a right-shifted curve representing decreased affinity.

Step 4: Final Answer:
Therefore, the only correct statement is (A).