Page 1 of 1: Partial pressures

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You will often hear that gases diffuse "along their partial pressure gradient". The pressure that a gas exerts in air is partial because that one gas only makes up part of the total pressure. For example, 21% of the total molecules in air are oxygen molecules. 21% is the fractional concentration of oxygen in air. Taking 760mmHg as total barometric pressure of inspired air at sea level, then the partial pressure of O2 is 160mmHg (or 0.21x 760). At altitude, the percentage that O2 represents of the total will be the same, but its partial pressure will be lower (because the total air pressure is lower). Partial pressures can be expressed in mmHg or kPa (1kPa is 7.5mmHg). Partial pressures are made more unapproachable by the abbreviations used to describe them. The notation, P is used for partial pressures in general, so that PO2 is the partial pressure of oxygen. By convention, PAO2 means that you are referring to the partial pressure of oxygen in alveolar gas and PaO2 in the arteries. Similarly PvO2 refers to the partial pressure of oxygen in the veins.

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