PO2 or Partial Oxygen Pressure: Importance and Factors Affecting Measurement

Oxygen Partial Pressure

Also known as PO2, it measures oxygen in the arterial blood. We estimate this to determine how well oxygen can pass from the lungs to the blood.

The PO2 test is used in association with a group of tests called arterial blood gas (ABG) tests, which also analyze carbon dioxide, bicarbonate (HCO3), and the pH level in red blood cells.

How to understand the partial pressures

Oxygen accounts for approximately 21 percent of the gases in our blood. The pressure of all the gases we breathe (oxygen, nitrogen, carbon dioxide) is about 760 mm Hg at sea level.

At higher altitudes, increases in atmospheric pressure result in a drop in the force of our blood gases. This includes the partial pressure of oxygen. The lower the levels, the less we can move the 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 about 4,000 to 10,000 feet above sea level.

Why it is essential to measure PO2

With each breath we take, oxygen reaches the lungs and is sent to the alveoli. The alveoli are where the transfer of oxygen and carbon dioxide occurs.


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

Because the partial pressure of oxygen is more significant in the alveoli than in the adjacent capillaries, it flows into the veins. In the same way, since the partial pressure of carbon dioxide is more significant in the capillaries than in the alveoli, it moves from the capillaries to the alveoli.

Any change in partial pressure can cause less oxygen to enter the blood and more carbon dioxide to accumulate in the blood.

None of these conditions is considered ideal. In some cases, as with hypoxemia, it can be dangerous.

Factors that affect PaO2 levels

A normal PO2 is between 75 and 100 mm Hg when the body functions normally. If your PO2 is below this, you are not getting enough oxygen.

Several factors can affect your PO2 levels. They include:

  • The partial pressure of oxygen in the air we inhale (high altitude environments versus low altitude environments).
  • Obstructions in the respiratory tree of our lungs (caused by conditions such as emphysema or pulmonary fibrosis).
  • The concentration of hemoglobin in our blood cells (having an iron deficiency means that your blood can cling to so many oxygen molecules).
  • The condition of your heart

Why is this oxygen measurement done?

  • PO2, as part of the ABG trial, is used to diagnose certain conditions or to evaluate an individual’s response to treatment, which includes:
  • We verify lung diseases such as asthma, cystic fibrosis, or chronic obstructive pulmonary disease (COPD).
  • You measure the acid-base level in your blood to see if you have kidney failure, heart failure, uncontrolled diabetes, or a severe infection.
  • Ensure you get the correct amount of oxygen if you are connected to a ventilator.
  • Evaluate how well you respond to the treatment of lung disease or trauma.