After breathing in (inhaling) oxygen enters the lungs and is transferred to a carrier protein called haemoglobin, which is found in red blood cells. These red blood cells carry the oxygen around the body and deliver it to all the tissues. Blood oxygenation is also called the blood oxygen saturation and describes the percentage of oxygen binding sites of blood haemoglobin that are actually bound to oxygen. Normal oxygenation levels are between 95 and 99 percent. If the blood oxygenation falls below 80 percent, the function of organs such as the brain and heart, will be compromised.