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...