Defining the Partial Pressure of Oxygen (Pa02)

Oxygen Makes Up 21 Percent of the Air We Breathe

man taking oxygen Photo©Wavebreakmedia

The partial pressure of oxygen refers scientifically to the pressure of oxygen amongst a mixture of gasses, but with medicine is most easily understood as a measurement of oxygen in arterial blood.

Measurement of PaO2 

You may hear your doctor talk about the partial pressure of oxygen, and wonder how it differs from oxygen saturation.  We'll get to that, but first, we'll talk about what partial pressure means.

Understanding Partial Pressures

The air that we breathe is mostly a combination of nitrogen, oxygen, and carbon dioxide, with oxygen making up roughly 21% of this. The pressure of all of the gasses we breathe together at sea level is around 760 mm Hg (millimeters of mercury.)  To figure out the partial pressure of the gasses in the blood you can multiply the pressure (whether at sea level or otherwise) by the percent of that gas present.

For example, at sea level, the partial pressure of oxygen in the air is 760 multiplied time 0.21 or 160 mm Hg  at elevations higher than sea level, the partial pressure of oxygen is lower.

This can help explain why some people have difficulty when traveling to regions at higher altitudes, or even on commercial air flights where the pressure in the cabin is equivalent to being at roughly 4,000 to 10,000 feet above sea level.


With each breath, oxygen is brought in through the mouth, passes down through the trachea, then through progressively smaller bronchi and bronchioles to the alveoli.

The alveoli are where the transfer of oxygen and carbon dioxide takes place in the lungs. The alveoli are lined up next to capillaries - the smallest blood vessels.  Oxygen passes from the alveoli by diffusion and into the capillaries while carbon dioxide passes from the capillaries into the alveoli to be exhaled.

Oxygen and carbon dioxide travel by diffusion - flowing down a gradient from a higher concentration to a lower concentration.Since the partial pressure of oxygen is higher in the alveoli than the capillaries, it flows into the capillaries. Likewise, since the partial pressure of carbon dioxide is higher in the capillaries than the alveoli, it diffusion from the capillaries into the alveoli.

Factors Affecting Oxygen Delivery to Tissues

It's important to talk about why different tests for oxygen level are used and the limitations of some of these.  It's easiest to do that by talking about the different variables that affect how much oxygen gets from the air we are breathing to our cells.  These include:

  • The partial pressure of oxygen of the air we inhale. (This decreases with altitude to the point at which the partial pressure of oxygen you would inhale at the top of Mount Everest is roughly a third of what you would inhale at sea level.)
  • How well oxygen travels from the air and down through the respiratory tree to the alveoli. (For example, this could be decreased if the airways are obstructed in some way.)
  • How well oxygen diffuses across the alveolar lining and across the capillary lining.
  • The concentration of hemoglobin in cells (If you have iron deficiency, your blood will not be able to hang on to as much oxygen.)
  • How strongly the hemoglobin hangs on to oxygen. (About 98.5% of oxygen is carried attached to hemoglobin and around 1.5% is carried dissolved in the blood.)
  • Cardiac output of the heart.

Normal Range of PaO2 (Partial Pressure of Oxygen) in the Alveoli and in the  Blood

The air around us has a partial pressure of oxygen around 160 mm Hg, but this changes as it is brought into the body and distributed.  In the alveoli, the partial pressure of oxygen is around 100 mm Hg and that of carbon dioxide is around 40 mm Hg.  In the cells of the body, the PaO2 is closer to 40. 

The range of normal for Pa02 is 75 - 100 mm Hg. If your Pa02 is less than this, it means you are not getting enough oxygen.

It is the differences in partial pressure between the capillaries and alveoli that drive oxygen from the alveoli into the capillaries in the lungs, and it is the difference between partial pressures of oxygen in the blood and that in the cells that drives the flow of oxygen from the tissue capillaries into cells.

Relationship PaO2 to Oxygen Saturation

PaO2 is a measure of all the oxygen in the blood - both that which is attached to hemoglobin, and that which is dissolved in the plasma. The majority of oxygen is carried in the blood attached to hemoglobin and only around 1.5% is dissolved in plasma.

In contrast, SaO2 (oxygen saturation) is a measure of how much hemoglobin is occupied by oxygen.

The difference between PaO2 and oxygen saturation becomes important only at lower oxygen concentrations due to something called the hemoglobin oxygen saturation curve.

On a graph with oxygen saturation on the left (the y-axis) and the partial pressure of oxygen on the right (the x-axis,) the curve increases rapidly and then levels off.  This means that at low partial pressures of oxygen, hemoglobin becomes rapidly saturated with oxygen, but this levels off with increasing partial pressures of oxygen. 

The amount of oxygen dissolved in the blood is directly proportional to the partial pressure of oxygen.  At higher partial pressures of oxygen, more oxygen will be dissolved in the blood.  If too much oxygen is dissolved (as can occur in scuba divers breathing in a different mixture of gasses) oxygen toxicity may result.

Unlike a scuba divers mixture of gasses, with an increase in PaO2, such as with supplemental oxygen or if you breathe more rapidly or deeply, the increase in PaO2 isn't followed by the same kind of increase in oxygen saturation when it gets close to 100%. It can't increase any further no matter how much oxygen is given.

But due to the curve, when the partial pressure of oxygen falls below a certain point, small decreases in the partial pressure of oxygen can cause major decreases in oxygen saturation.  This can help explain why, when someone is oxygen dependent, they can get in trouble rapidly when they run out of oxygen (or take a flight without adjustments in their oxygen level.)  On the other side of this slope is that it takes very little oxygen to get someone out of trouble.  For example, using a Venturi oxygen mask to deliver 24% oxygen instead of the 21% of room air may quickly help someone who is struggling to breathe feel better.  This becomes important in people with hypercapnic respiratory failure -those with COPD who retain carbon dioxide.  In that setting increasing arterial oxygen concentration could result in further oxygen retention.

Conditions with Decreased PaO2

A low-level of oxygen in the blood is referred to as hypoxemia.  When hypoxemia results in a low -evel of oxygen in tissues it is then referred to as hypoxia.  Tissue hypoxia results in tissue damage, and if not corrected, eventually cell death.


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.

Collins, J., Rudenski, A., Gibson, J., Howard, L., and R. O’Driscoll. Relating oxygen partial pressure, saturation and content: the haemoglobin oxygen dissociation curve. Breathe. 2015. DOI: 10.1183/20734735.001415

Feller-Kopman, D., and R. Schwartzstein. Mechanisms, causes, and effects of hypercapnia. UptoDate. 07/30/15.

U.S. National Library of Medicine. Medline Plus. Blood gases. Updated 08/25/14.

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