PaO2

If you dissolve a pinch of sugar in water, you can measure how 'sugary' the water is. PaO2 is the equivalent measure for oxygen dissolved in arterial blood — the pressure exerted by the free oxygen molecules floating loose in the plasma, after the blood has been freshly oxygenated by the lungs. The small 'a' stands for arterial.

Crucially, PaO2 measures only the oxygen dissolved in the watery part of the blood, which is a tiny fraction of the total. The great majority of oxygen rides bound to hemoglobin inside red cells and is not counted by PaO2 directly. Even so, this dissolved fraction matters enormously, because it is the dissolved oxygen pressure that determines how fully hemoglobin loads up — the link described by the oxygen–hemoglobin dissociation curve. A typical healthy value breathing room air at sea level is roughly 80 to 100 mmHg.

Because it reflects how well the lungs are transferring oxygen into the blood, PaO2 is a cornerstone of the arterial blood gas test. A low value defines hypoxemia and prompts a search for its cause — mismatch of air and blood flow, shunt, a thickened membrane, or simply thin air at altitude. A caveat: PaO2 must always be interpreted alongside the inspired oxygen the person is breathing, since supplemental oxygen raises it.