Pathophysiology
Clinical meaning
The oxyhemoglobin dissociation curve describes the relationship between the partial pressure of oxygen (PaO2) and hemoglobin oxygen saturation (SaO2), reflecting the cooperative binding kinetics of the hemoglobin molecule. Hemoglobin is a tetrameric protein consisting of four globin subunits (two alpha, two beta in adult HbA), each containing a heme group with a central ferrous iron (Fe2+) atom that reversibly binds one oxygen molecule. The curve is sigmoid (S-shaped) because of cooperative binding: when the first oxygen molecule binds to one heme group, it induces a conformational change from the T-state (tense, low affinity) to the R-state (relaxed, high affinity), progressively increasing the affinity of the remaining unoccupied heme groups. This cooperativity produces the steep middle portion of the curve (PaO2 20-60 mmHg) where small changes in PaO2 cause large changes in saturation, and the flat upper plateau (PaO2 >70 mmHg) where hemoglobin is nearly fully saturated and further PaO2 increases add little additional oxygen. The clinical significance of the curve centers on the P50 -- the PaO2 at which hemoglobin is 50% saturated (normal P50 = 26.6 mmHg). A right...