Which of the following is the most common physiological factor causing a rightward shift of the oxygen-hemoglobin dissociation curve?

Explanation

## Rightward Shift of the Oxygen-Hemoglobin Dissociation Curve **Key Point:** A rightward shift decreases hemoglobin's affinity for oxygen, facilitating oxygen unloading to tissues. This is the most physiologically important adaptation in metabolically active tissues. ### Physiological Factors Causing Rightward Shift (Bohr Effect & Beyond) | Factor | Effect on Curve | Mechanism | |--------|-----------------|----------| | **Increased 2,3-DPG** | Rightward shift | Binds to central cavity of deoxyHb, stabilizes T state | | Increased temperature | Rightward shift | Reduces Hb-O₂ affinity | | Decreased pH (acidosis) | Rightward shift | H⁺ ions stabilize deoxyHb | | Increased PaCO₂ | Rightward shift | CO₂ forms carbaminohemoglobin | | Increased 2,3-DPG | **Most common physiological** | Chronic hypoxia, high altitude, anemia | **High-Yield:** The mnemonic for rightward shift factors is **CADET, go LEFT** (but reversed): - **C**O₂ ↑ - **A**cid ↓ (pH ↓) - **D**PG (2,3-DPG) ↑ - **E**xercise (↑ temp, ↑ CO₂, ↑ acid) - **T**emperature ↑ ### Why Increased 2,3-DPG Is Most Common 1. **Chronic physiological adaptation:** Occurs in chronic hypoxia, high altitude, anemia, and chronic lung disease—all common clinical scenarios. 2. **Tissue oxygen delivery:** 2,3-DPG is the primary regulator of oxygen unloading in chronically stressed states. 3. **Mechanism:** 2,3-DPG binds to the central cavity between the β-globin chains of deoxygenated hemoglobin, stabilizing the T (tense) state and reducing O₂ affinity. **Clinical Pearl:** Patients living at high altitude develop elevated 2,3-DPG within hours to days, enabling better oxygen delivery despite lower PaO₂. This is why the rightward shift due to 2,3-DPG is the most common physiological adaptation. ### Distinction from Other Factors - **Temperature:** Rightward shift occurs, but is not a chronic physiological regulator in the same sense. - **pH and PaCO₂:** Part of the Bohr effect; important acutely but 2,3-DPG is the primary chronic adaptation. [cite:Guyton & Hall Textbook of Medical Physiology Ch 41]