The oxygen status of arterial human blood.

The oxygen status of arterial human blood is described at least by four variables: Oxygen partial pressure (pO2, mmHg), oxygen saturation (sO2, %), hemoglobin content (cHb, g/dL) and oxygen content (cO2, mL/dL). Beside perfusion, however, the oxygen supply of all organs is decisively determined by the mean capillary pO2 which itself is primarily dependent on the arterial cO2. Therefore, the oxygen availability (cardiac output x caO2, mL/min) may be described by the cO2 value in arterial blood or those variables who determine the latter one. The diagnostic significance of the O2 variables of the oxygen status consequently increases in the order of pO2, sO2 (cHb) and cO2. In arterial blood, oxygen partial pressure is the result of O2 diffusion within the lungs into the blood (lung function). Oxygen saturation describes the portion of chemically bound oxygen expressed as O2Hb in relation to total Hb (Hb + O2Hb + COHb + MetHb). Oxygen content is the total amount of oxygen in blood chemically bound plus physically dissolved. Under pathophysiological conditions the diagnostic significance becomes very clear. Disturbances of lung function decreases all three variables, pO2 (hypoxia), sO2 (hypoxygenation) and cO2 (hypoxemia), to produce hypoxic hypoxemia. Carbon monoxide poisoning or methemoglobin formation decreases two variables, sO2 and cO2, where the pO2 remains normal and results in toxic hypoxemia. Anemia with a decrease in the hemoglobin content lowers cO2 only, while pO2 and sO2 remain normal (anemic hypoxemia).