## Oxygen-Hemoglobin Dissociation Curve **Key Point:** The P₅₀ (partial pressure at which hemoglobin is 50% saturated) is approximately 26–27 mmHg under standard conditions (pH 7.40, temperature 37°C, PaCO₂ 40 mmHg). ### Standard Reference Points on the Curve | PaO₂ (mmHg) | SaO₂ (%) | Clinical Significance | | --- | --- | --- | | 26–27 | 50 | P₅₀; venous blood typical value | | 40 | 75 | Mixed venous oxygen | | 60 | 90 | Steep portion of curve | | 100 | 97–98 | Arterial blood (normal) | **High-Yield:** The P₅₀ of 26 mmHg is a critical reference point in respiratory physiology. At this pressure, hemoglobin releases exactly half its bound oxygen to tissues — this is the point of maximum cooperativity and sensitivity to physiological changes. **Clinical Pearl:** Factors that **shift the curve rightward** (increase P₅₀, decrease affinity) include ↑ temperature, ↓ pH (acidosis), ↑ PaCO₂, ↑ 2,3-DPG. Factors that **shift leftward** (decrease P₅₀, increase affinity) include ↓ temperature, ↑ pH (alkalosis), ↓ PaCO₂, ↓ 2,3-DPG, fetal hemoglobin, carbon monoxide. **Mnemonic:** **CADET** — CO₂, Acid, 2,3-DPG, Exercise (heat), Temperature — all shift the curve **rightward** (away from hemoglobin, toward tissues).
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