Figure 32.20: Loading and Unloading of O2 by Human Hemoglobin This curve—known as an oxygen equilibrium curve—shows the chemical behavior of human hemoglobin under conditions in arterial blood. Hemoglobin almost instantly adjusts its O2 binding to match changes in the prevailing O2 partial pressure, taking up or releasing O2 as needed. The vertical green arrows show how much O2 is unloaded by hemoglobin as it flows from the lungs through the systemic tissues during rest and exercise. The blood O2 partial pressure is expressed in millimeters of mercury (mm Hg) of pressure.