The lungs transfer oxygen from the air into the bloodstream. This oxygenated (arterial) blood is then pumped by the heart to all organ systems. The arterial oxygen blood level can be measured by drawing blood from an artery (arterial blood gas measurement). This accurately measures the oxygen and carbon dioxide (waste gas of metabolism) and levels of each in the blood.
An oximeter, usually attached to the finger, shines two separate light beams into the blood circulating in the small vessels, i.e., capillaries. These light beams reflect the amount of oxygen in the blood, expressed as a percentage along with the pulse rate (Figure 1).