Blood
About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at sea-level pressure is chemically combined with the hemoglobin. About 1.5% is physically dissolved in the other blood liquids and not connected to hemoglobin. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species (for exceptions, see below). Hemoglobin has an oxygen binding capacity between 1.36 and 1.40 ml O per gram hemoglobin, which increases the total blood oxygen capacity seventyfold, compared to if oxygen solely were carried by its solubility of 0.03 ml O per liter blood per mm Hg partial pressure of oxygen (about 100 mm Hg in arteries).
Oxygenation (environmental)
Supersaturation of oxygen (i.e. saturation levels above 100%) can occur naturally. The most common cause is oxygen production by photosynthetically active species such as plants and algae. According to Henry's law, the equilibrium oxygen concentration is proportional to the partial pressure of oxygen gas. As air contains about 21% oxygen the equilibrium concentration of pure oxygen gas corresponds to nearly 500% air saturation. The other reason is that the oxygen concentration can be slow to adjust to changes in the environment. A rapid increase in temperature can reduce the equilibrium concentration of oxygen to a value below the actual concentration in the water, giving raise to more than 100% saturation until the system has had time to equilibrate through diffusion. Supersaturation can sometimes be harmful for organisms and cause decompression sickness.