What does the partial pressure of oxygen (PO2) measure?

The partial pressure of oxygen (PO2) specifically measures the amount of unbound, or free, oxygen molecules that are dissolved in the plasma of the blood. This concept is crucial in understanding how oxygen is transported and utilized in the body, as the PO2 directly influences the diffusion of oxygen from the alveoli in the lungs into the bloodstream, and subsequently into the tissues.

In this context, PO2 is integral to assessing a person's respiratory efficiency and is often measured in arterial blood gas (ABG) tests. This measure is distinct from oxygen saturation, which reflects the percentage of hemoglobin molecules that are bound with oxygen. While both factors are important for understanding oxygen transport, they represent different physiological parameters. The other options focus on different aspects of oxygen measurement and transport, further defining the unique role that PO2 plays in clinical assessments.