All of the following statements regarding oxygen transport in blood are correct EXCEPT:

## Oxygen Transport in Blood — Correcting the Misconception ### The Oxygen-Hemoglobin Dissociation Curve **Key Point:** The relationship between PaO₂ and hemoglobin saturation is **non-linear** and **sigmoidal**, NOT linear. This S-shaped curve is one of the most critical concepts in respiratory physiology. ### Why the Correct Answer is Wrong The oxygen-hemoglobin dissociation curve exhibits **cooperative binding** (allosteric effect): - At **low PaO₂** (< 40 mmHg): saturation increases slowly with rising PaO₂ - At **mid-range PaO₂** (40–60 mmHg): saturation increases steeply (the steep portion of the S-curve) - At **high PaO₂** (> 80 mmHg): saturation plateaus and increases minimally This non-linearity is **physiologically advantageous**: - In the lungs (PaO₂ ≈ 100 mmHg): hemoglobin loads oxygen efficiently - In tissues (PaO₂ ≈ 40 mmHg): hemoglobin releases oxygen readily ### Verification of Other Statements | Statement | Status | Explanation | |-----------|--------|-------------| | Dissolved O₂ ≈ 3 mL/L | **Correct** | Only 0.3 mL/100 mL plasma; hemoglobin carries ~97% of O₂ | | 2,3-DPG rightward shift | **Correct** | Decreases affinity, facilitates tissue unloading | | SaO₂ at PaO₂ 100 mmHg | **Correct** | Normal arterial saturation is 97–98% | **High-Yield:** The sigmoid shape is due to **positive cooperativity**—binding of one O₂ molecule increases affinity for the next, explaining the steep middle section. **Clinical Pearl:** In chronic hypoxia (e.g., high altitude, COPD), increased 2,3-DPG shifts the curve rightward, lowering saturation at any given PaO₂ but improving tissue oxygen delivery—a compensatory mechanism. **Mnemonic:** **SHIFT RIGHT = UNLOAD TIGHT** - Rightward shift → decreased affinity → easier unloading in tissues - Causes: ↑ 2,3-DPG, ↑ H⁺ (acidosis), ↑ CO₂, ↑ temperature