Blood Oxygen Saturation
Blood Oxygen Saturation (SpO2) is the percentage of hemoglobin molecules in arterial blood that are carrying oxygen. The "Sp" prefix indicates measurement via pulse oximetry rather than direct blood sampling. Normal SpO2 values at sea level are 95β100% in healthy individuals. The metric reflects how effectively the respiratory system is oxygenating the blood.
Typical Adult Ranges
%Based on population studies. Individual needs vary by age and health status.
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Key Takeaways
Represents percentage of hemoglobin carrying oxygen.
Normal values at sea level are 95β100%.
Lower values expected at altitude.
Overnight dips may indicate breathing disturbances.
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How It's Measured
Oura measures SpO2 using reflectance pulse oximetry.
Common Influences
Altitude: Higher elevation reduces atmospheric oxygen pressure, lowering SpO2.
Deep Dive
SpO2 represents the saturation state of hemoglobin with oxygen. It indicates whether the lungs and circulatory system are successfully delivering oxygen to the bloodstream.
A useful framing is that SpO2 indicates the percentage of oxygen-carrying capacity being utilized. At 98% saturation, nearly all hemoglobin binding sites carry oxygen; at 90%, a meaningful proportion are not. The metric reflects the endpoint of the respiratory process: oxygen moving from air into blood.
Oxygen enters the blood through gas exchange in the lungs. Hemoglobin in red blood cells binds oxygen in the pulmonary capillaries and releases it in tissues. The saturation level depends on oxygen partial pressure in the lungs, hemoglobin binding characteristics, and cardiopulmonary function.
At sea level, healthy lungs easily achieve >95% saturation. Lower saturation can result from respiratory conditions (asthma, COPD, pneumonia), sleep-disordered breathing (apnea events), high altitude (reduced oxygen partial pressure), or cardiovascular issues affecting oxygen delivery.
The oxyhemoglobin dissociation curve describes the relationship between oxygen partial pressure and saturation. The curve is sigmoidal: saturation stays high until oxygen pressure drops substantially, then falls more rapidly. This means small changes in the 95β100% range may reflect larger changes in underlying oxygen delivery.