 Arterial oxygen partial pressure

Arterial oxygen partial pressure, $P_{\mathrm{a}{O}_2}$, is the oxygen tension assessed in an arterial blood sample: the oxygen partial pressure of a gas phase in equilibrium with that blood. It relates to dissolved oxygen, rather than measuring all oxygen bound to hemoglobin.^[ref] The arterial qualifier identifies the sampled circulation; a venous measurement answers a different sampling question.

A simple equilibrium model writes dissolved oxygen concentration aswhere the solubility coefficient $\alpha$ depends on the specified fluid and temperature. For an original illustration, choose $\alpha =0.003$ mL O${}_2$/(dL·mmHg). Pressures 100 and 200 mmHg then correspond to 0.30 and 0.60 mL/dL dissolved oxygen. Those stipulated model concentrations do not include the carrier-bound contribution.

Oxygen saturation instead describes occupied binding sites, and arterial oxygen content also depends on how much carrier is present. Two samples can therefore have the same $P_{\mathrm{a}{O}_2}$ while differing in hemoglobin concentration and total oxygen content. A pressure reading alone does not count the available binding sites or determine oxygen delivery by the blood flow.

Blood gas analysis supplies a sample-specific observation to interpret alongside the inspired gas and respiratory state. The measurement should identify the compartment, temperature convention and sampling conditions. An arterial result, an alveolar estimate and an inspired oxygen fraction are related steps in oxygen transport, not three names for the same quantity.