Understanding the Partial Pressure of Oxygen (PaO2)

A Vital Diagnostic Tool to Help Evaluate Blood Gases

Oxygen therapy
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The partial pressure of oxygen, also known as PaO2, is a measurement of oxygen in arterial blood. It shows how well oxygen is able to move from the lungs to the blood.

The PaO2 is one of the components measured in the arterial blood gas (ABG) test, which also reports carbon dioxide, bicarbonate (HCO3), and the pH level in red blood cells. See what this means, the factors that influence it, and what it can tell your doctor about your health.

Normal Range of PaO2

When the body is functioning normally, the PaO2 is between 75 and 100 mm Hg at sea level. As altitudes increase, normal values decrease (as the PaO2 decreases).

Understanding Partial Pressures

Oxygen makes up roughly 21 percent of the gases in your blood. The pressure of all of the gases you breathe (oxygen, nitrogen, carbon dioxide) is roughly 760 millimeters of mercury (mm Hg) at sea level.

At higher altitudes, decreases in atmospheric pressure result in a drop in the pressure of your blood gases, including the partial pressure of oxygen. The lower the levels go, the less you are able to move oxygen from your lungs to your 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 you take, oxygen is brought into the lungs and is delivered to the alveoli. The 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.

Decreases in PaO2 (partial pressure of oxygen in the alveoli) result in less O2 in the blood and increases in PaO2 result in increases of O2 in the blood.

Factors Affecting PaO2 Levels

If your PaO2 is below the normal range, 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 you inhale (high-altitude versus low-altitude environments)
  • Obstructions in the respiratory tree of your lungs (caused by conditions like emphysema or pulmonary fibrosis)
  • The concentration of hemoglobin in your 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
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