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.
Breathing
The primary purpose of breathing is to bring atmospheric air (in small doses) into the alveoli where gas exchange with the gases in the blood takes place. The equilibration of the partial pressures of the gases in the alveolar blood and the alveolar air occurs by diffusion. At the end of each exhalation the adult human lungs still contain 2,500–3,000 mL of air, their functional residual capacity or FRC. With each breath (inhalation) only as little as about 350 mL of warm, moistened atmospheric is added, and well mixed, with the FRC. Consequently, the gas composition of the FRC changes very little during the breathing cycle. Since the pulmonary capillary blood equilibrates with this virtually unchanging mixture of air in the lungs (which has a substantially different composition from that of the ambient air), the partial pressures of the arterial blood gases also do not change with each breath. The tissues are therefore not exposed to swings in oxygen and carbon dioxide tensions in the blood during the breathing cycle, and the peripheral and central chemoreceptors do not need to "choose" the point in the breathing cycle at which the blood gases need to be measured, and responded to. Thus the homeostatic control of the breathing rate simply depends on the partial pressures of oxygen and carbon dioxide in the arterial blood. This then also maintains the constancy of the pH of the blood.