Figure 17-25 Oxygen-hemoglobin dissociation curve. Four different ordinates are shown as a function of oxygen partial pressure (the abscissa). In order from right to left, they are saturation (%); O₂ content (mL of O₂ /0.1 L of blood); O₂ supply to peripheral tissues (mL/min); and O₂ available to peripheral tissues (mL/min), which is O₂ supply minus the approximately 200 mL/min that cannot be extracted below a partial pressure of 20 mm Hg. Three points are shown on the curve: a, normal arterial pressure; , normal mixed venous pressure; and P₅₀ , the partial pressure (27 mm Hg) at which hemoglobin is 50% saturated.