Regarding factors that affect the oxygen dissociation curve of hemoglobin, all of the following cause a rightward shift (increased P50) EXCEPT:

Explanation

## Oxygen Dissociation Curve: Factors Affecting P50 ### Definition of Rightward vs Leftward Shift **Key Point:** A rightward shift of the oxygen dissociation curve means increased P50 (higher partial pressure of O₂ required to achieve 50% Hb saturation), indicating *decreased* oxygen affinity and *increased* oxygen release to tissues. ### Factors Causing Rightward Shift (Decreased Affinity) **Mnemonic:** CADET, face Right! — **C**O₂ ↑, **A**cid ↑, **2,3-DPG** ↑, **E**xercise/temperature ↑, **T**emperature ↑ | Factor | Effect | Mechanism | |--------|--------|----------| | ↑ 2,3-DPG | Rightward | Stabilizes T (tense) state; binds to central cavity of deoxyHb | | ↓ pH (acidosis) | Rightward | Bohr effect; H⁺ stabilizes deoxyHb | | ↑ Temperature | Rightward | Weakens Hb-O₂ bonds; favors heat dissociation | | ↑ CO₂ | Rightward | Bohr effect + carbaminohemoglobin formation | ### Analysis of Options **Option 1: Increased 2,3-DPG** — ✓ Causes rightward shift (allosteric effector) **Option 2: Decreased pH (acidosis)** — ✓ Causes rightward shift (Bohr effect) **Option 3: Increased temperature** — ✓ Causes rightward shift (thermal destabilization of O₂ binding) **Option 4: Decreased CO₂ tension** — ✗ This causes a **LEFTWARD** shift - Lower CO₂ → less carbaminohemoglobin formation - Lower CO₂ → higher pH (respiratory alkalosis) → increased Hb-O₂ affinity - Result: decreased P50 (curve shifts left, oxygen *clings* to Hb) **High-Yield:** The question tests understanding that decreased CO₂ is the ONLY option that causes a leftward shift, making it the exception to the rightward-shift pattern. **Clinical Pearl:** In chronic hypoxemia (high altitude), increased 2,3-DPG rightward-shifts the curve to enhance oxygen unloading at tissues, despite lower arterial PO₂.