Understanding Oxygen Saturation

pulse monitor on finger
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Oxygen saturation—sometimes referred to as O sats, or simply, sats—refers to the extent to which hemoglobin is saturated with oxygen. Hemoglobin is an element in the blood that binds with oxygen to carry it through the bloodstream to the organs, tissues, and cells of the body. Normal oxygen saturation is usually between 96 percent and 98 percent. 

Each of our red blood cells contains 4 molecules of hemoglobin.

Iron, which is present in hemoglobin, is what oxygen binds to after diffusing from the alveoli in the lungs and into the capillaries of the lungs.  Most of the time the hemoglobin is fully saturated.

Oxygen saturation is dependent upon:

  • Oxygen availability.
  • Gas exchange in the lungs: The ability of oxygen to reach the alveoli, and once in the alveoli, to diffuse through the walls of the alveoli and capillary to reach the red blood cells.
  • The concentration of hemoglobin in red blood cells.
  • The affinity of hemoglobin for oxygen, in other, words how strongly hemoglobin attracts oxygen.

Measuring Oxygen Saturation

Oxygen saturation is most commonly measured by either:

  • Arterial blood gasses: The value obtained from arterial blood gasses or ABG's (SaO2) describes the oxygen saturation of arterial blood, and is obtained by drawing blood from an artery such as the radial artery in the wrist or the femoral artery in the groin.
  • Pulse oximetry: The value obtained from peripheral capillary blood using pulse oximetry (SpO2) often closely reflects the levels that would be found in arterial blood. Pulse oximetry has the advantage of being a non-invasive test, which uses a probe attached to a finger or earlobe or other regions of the body that reads the wavelengths of light reflected from the blood. Not only are pulse oximeters a standard now for monitoring people in the hospital, but people can even follow their own saturation via the use of wearable technology.

    Decreased Oxygen Saturation

    A drop in oxygen saturation levels is referred to desaturation, or hypoxemia, and can be caused by any changes or damage in the variables above.

    • A change in oxygen availability can be caused by a decreased concentration of oxygen in inspired air such as at higher altitudes and when flying in an airplane.
    • Problems with gas exchange causing a decrease in oxygen saturation can include anything that reduces the ability of oxygen to travel from the outside air down into the alveoli of our bodies or during the process of oxygen transfer from the alveoli into the capillaries of the blood.
    • Decreased oxygen saturation may result from a lower concentration of hemoglobin, such as in iron deficiency anemia.
    • A decreased affinity of hemoglobin for oxygen may occur when there is something else present which binds more strongly to hemoglobin than does oxygen, such as in carbon monoxide poisoning in which the compound carboxyhemoglobin is formed.

    Hypoxemia and Hypoxia

    The terms hypoxemia and hypoxia are often used interchangeably but mean quite different things.  Hypoxemia is the term which defines a decreased oxygen concentration in the blood. Hypoxia, in turn, describes the consequences that occur due to hypoxemia.

     When cells do not get enough oxygen, they may adapt if the deficiency is small. However, in larger deficiencies, the result is cell damage followed by cell death.

    Hypoxia is often caused by hypoxemia, but may also occur when:

    • There is anemia because there are too few red blood cells so even fully oxygenated blood doesn't bring enough oxygen to tissues.
    • There is inadequate blood flow so even fully oxygenated blood does not reach the tissues.
    • The tissues are unable to use fully oxygenated blood that is delivered.
    • The tissues require even more oxygenated blood than can be delivered, such as in severe infections.

      Treating Hypoxemia and Hypoxia Due to Hypoxemia

      When oxygen saturation drops below a certain level, supplemental oxygen therapy is usually needed, and sometimes emergently. It is also important to determine the cause of a low oxygen saturation. Treating the underlying cause is then the primary goal of treatment.


      Collins, J., Rudenski, A., Gibson, J., Howard, L., and R. O’Driscoll. Relating oxygen partial pressure, saturation and content: the haemoglobin–oxygen dissociation curve. Breathe. 2005. DOI: 10.1183/20734735.001415.

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