Partial pressure

In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of the gases in the mixture (Dalton's Law). Partial pressure can be highlighted with a thought experiment using semipermeable pistons, as underlined by Boltzmann, Gibbs and Planck.

In respiratory physiology, the partial pressure of a dissolved gas in liquid (such as oxygen in arterial blood) is also defined as the partial pressure of that gas as it would be undissolved in gas phase yet in equilibrium with the liquid. This concept is also known as blood gas tension. In this sense, the diffusion of a gas liquid is said to be driven by differences in partial pressure (not concentration). In chemistry and thermodynamics, this concept is generalized to non-ideal gases and instead called fugacity. The partial pressure of a real gas is a measure of its thermodynamic activity. Gases dissolve, diffuse, and react according to their partial pressures (which appear in reaction quotient or equilibrium constant of gas reactions) and not according to their concentrations in a gas mixture or as a solute in solution. This general property of gases is also true in chemical reactions of gases in biology.