"Oxygen dissolved in the plasma will first enter the red blood cell. As the dissolved oxygen concentration inside the cell increases, hemoglobin will begin to bind the oxygen. Each hemoglobin molecule is a tetramer consisting of four subunits, and each subunit binds one oxygen molecule. Therefore, each hemoglobin molecule can bind a total of four oxygen molecules. Once the oxygen is bound to hemoglobin, it no longer contributes to Po2. Thus, hemoglobin is like a "sink" that can take up oxygen until each hemoglobin molecule is fully (100%) saturated with four oxygen molecules. "