Oxygen is carried in the blood in two forms: (1) dissolved in plasma and RBC water (about 2% of the total) and (2) reversibly bound to hemoglobin (about 98% of the total). At physiological PO2 (40 &#60; PO2 &#60; 100 mm Hg), only a small amount of oxygen is dissolved in plasma since oxygen has such a low solubility.