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 shift (increased P50) decreases hemoglobin-oxygen affinity, facilitating oxygen unloading to tissues. Factors causing right shift include acidosis (Bohr effect: H+ ions protonate histidine residues on hemoglobin, stabilizing the T-state), hypercapnia (CO2 forms carbamino compounds with terminal amino groups), hyperthermia (increased molecular kinetic energy weakens O2-hemoglobin bonds), and increased 2,3-diphosphoglycerate (2,3-DPG, an erythrocyte glycolytic intermediate that binds the central cavity of deoxyhemoglobin, stabilizing the T-state). A left shift (decreased P50) increases hemoglobin-oxygen affinity, impairing oxygen release to tissues. Left-shift factors include alkalosis, hypothermia, decreased 2,3-DPG (depleted in banked blood within 1-2 weeks of storage), carbon monoxide (CO binds hemoglobin with 200-250x greater affinity than oxygen, locking remaining heme groups in the R-state), and fetal hemoglobin (HbF has gamma chains that bind 2,3-DPG poorly, resulting in higher O2 affinity -- advantageous for transplacental oxygen transfer but clinically relevant in sickle cell disease treatment with hydroxyurea). Methemoglobinemia represents a special case: oxidation of Fe2+ to Fe3+ (ferric iron) renders the affected heme unable to bind oxygen at all, while simultaneously left-shifting the remaining functional heme groups, creating a dual impairment in oxygen delivery.