Oxygen saturation (medicine)
Oxygen saturation is the fraction of oxygen-saturated hemoglobin relative to total hemoglobin (unsaturated + saturated) in the blood. The human body requires and regulates a very precise and specific balance of oxygen in the blood. Normal blood oxygen levels in humans are considered 95–100 percent. If the level is below 90 percent, it is considered low resulting in hypoxemia. Blood oxygen levels below 80 percent may compromise organ function, such as the brain and heart, and should be promptly addressed. Continued low oxygen levels may lead to respiratory or cardiac arrest. Oxygen therapy may be used to assist in raising blood oxygen levels. Oxygenation occurs when oxygen molecules () enter the tissues of the body. For example, blood is oxygenated in the lungs, where oxygen molecules travel from the air and into the blood. Oxygenation is commonly used to refer to medical oxygen saturation.
Oxygen
The common allotrope of elemental oxygen on Earth is called dioxygen, , the major part of the Earth's atmospheric oxygen (see Occurrence). O has a bond length of 121 pm and a bond energy of 498 kJ/mol, which is smaller than the energy of other double bonds or pairs of single bonds in the biosphere and responsible for the exothermic reaction of O with any organic molecule. Due to its energy content, O is used by complex forms of life, such as animals, in cellular respiration. Other aspects of are covered in the remainder of this article.