The Definition of Partial Pressure of Oxygen (PaO2)

A Vital Diagnostic Tool to Help Evaluate Blood Gasses

Oxygen therapy
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The partial pressure of oxygen, also known as PaO2, is the measurement of oxygen in arterial blood. We measure this in order to determine how well oxygen is able to move from the lungs to the blood.

The PaO2 test is used in association a battery of tests called the arterial blood gasses (ABG) test, which also looks at carbon dioxide, bicarbonate (HCO3), and the pH level in red blood cells.

Understanding Partial Pressures

Oxygen makes up roughly 21 percent of the gasses in our blood.

The pressure of all of the gasses we breathe (oxygen, nitrogen, carbon dioxide) is roughly 760 mm Hg at sea level.

At higher altitudes, increases in atmospheric pressure result in a drop in the pressure of our blood gasses. This includes the partial pressure of oxygen. The lower the levels go, the less we are able to move oxygen from our lungs to our blood.

This helps explain why some people have trouble breathing at higher altitudes, or even on commercial flights where the pressure in the cabin is equivalent to being at roughly 4,000 to 10,000 feet above sea level.

Why Measuring PaO2 Is Important

With each breath we take, oxygen is brought into the lungs and is delivered to the alveoli. Alveoli are where the transfer of oxygen and carbon dioxide takes place.

Partial pressure is the dynamic that explains why oxygen moves from the alveoli into the blood and why carbon dioxide moves from the blood into the alveoli.

Because the partial pressure of oxygen is higher in the alveoli than the adjacent capillaries, it flows into the capillaries. Likewise, since the partial pressure of carbon dioxide is higher in the capillaries than the alveoli, it moves from the capillaries into the alveoli.

Any changes in partial pressure can result in less oxygen getting into the blood and more carbon dioxide accumulating in the blood.

Neither of these conditions is considered ideal. In some cases, as with hypoxemia, it can be dangerous.

Factors Affecting PaO2 Levels

When the body is functioning normally, a normal PaO2 is between 75 and 100 mm Hg. If your PaO2 is below this, it means you are not getting enough oxygen.

There are a number of factors can affect your PaO2 levels. They include:

  • the partial pressure of oxygen of the air we inhale (high-altitude versus low-altitude environments)
  • obstructions in the respiratory tree of our lungs (caused by conditions like emphysema or pulmonary fibrosis)
  • the concentration of hemoglobin in our blood cells (having an iron deficiency means that your blood is able to hang on to as many oxygen molecules)
  • the condition of your heart

Why the PaO2 Test Is Performed

The PaO2, as part of the ABG assay, is used to diagnose certain conditions or to assess an individual's response to treatment, including:

  • checking for lung diseases such as asthma, cystic fibrosis, or chronic obstructive pulmonary disease (COPD).
  • measuring the acid-base level in your blood if you have kidney failure, heart failure, uncontrolled diabetes, or a severe infection.
  • ensuring you are getting the right amount of oxygen if you are on a ventilator.
  • assessing how well you are responding to treatment for a lung disease or trauma.


Carreau, A.; El Hafny-Rahbi, B.; Matejuk, A.; Grillon, C., and C. Kieda. "Why is the partial oxygen pressure of human tissues a crucial parameter? Small molecules and hypoxia." Journal of Cellular and Molecular Medicine. 2011; 15(6):1239-53.

U.S. National Library of Medicine. Medline Plus. "Blood gasses." MedLine Plus. Bethesda, Maryland; Updated August 25, 2014.