What Is a Normal Resting Blood Oxygen Level?

Maintaining a normal blood oxygen level is critical to maintaining the metabolic functions of the body on a daily basis. Oxygen, as you know, is essential to life and when cells are deprived of oxygen, they will eventually die.

Explaining the Oxygen Delivery System

Oxygen is taken in from the air through the mouth and nose and drawn into the lungs.  Capillaries surround the bronchial tubes inside the lungs.  Oxygen diffuses into the blood vessels, and attaches to the red blood cell, which transports the chemical to cells throughout the body.  Cells use oxygen to create a molecule called ATP, which is energy needed for metabolism and other cellular processes.

How is Blood Oxygen Measured?

Oxygen saturation is measured by a pulse oximeter, a device placed on the finger that uses light absorption to indicate the level of saturated hemoglobin, the protein on the red blood cell that stores oxygen during transport. Oxygen levels can also be determined through a blood test called an arterial blood gas, or ABG.  This test measures the partial pressure of many blood gasses, including oxygen. 

Normal Blood Oxygen Levels

Healthy people have a normal range of oxygen saturation from 95% – 100%, although levels down to 90% are common and not usually cause for alarm.  During exercise, the increased oxygen consumption may reduce blood saturation, but the increased breathing rate and blood flow compensate for this. Oxygen levels remain within the established normal range.

Low Oxygen Blood Levels (Hypoxemia)

Hypoxemia is the condition of not having enough oxygen in the blood. Causes of hypoxemia include low inspired oxygen levels, alveolar hypoventilation, ventilation-perfusion inequality, and impairment of the oxygen diffusion across the cell membrane. Ventilation-perfusion inequality is a common cause of hypoxemia in people with lung disease such as COPD, emphysema, sleep apnea, acute respiratory distress syndrome, asthma, or interstitial lung disease.  It can also occur with cardiac problems, such as congenital heart disease and low cardiac output, where the heart does not adequately pump blood throughout the body.

Cigarette smoking can also cause low blood oxygen levels even without the presence of a lung condition.  Cigarette smoke contains carbon monoxide, which competes with oxygen on the hemoglobin of the red blood cell.  Because hemoglobin saturated with CO (carbon monoxide) can not be differentiated from that with oxygen, a pulse oximetry test may indicate the blood level is normal, when in fact the oxygen levels are low.


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