Partial Pressure

The percentage of a gas in a mixture does not accurately convey the quantity of the gas present. For example, oxygen has the same percentage at sea level and at the top of Mount Everest, but in the latter case the actual quantity of oxygen is much lower. Therefore we use the partial pressure (P) of a gas as a surrogate measure of the number of molecules.

The partial pressure of a gas is a measure of the concentration of the gas in the medium or tissue in which it exists e.g. arterial blood or air.

Dalton’s law

Each gas in a mixed gas environment produces its own partial pressure with the result that the sum of the pressures of the different gases provides the total pressure created by the mixture. This is defined in Dalton’s law of partial pressure.

The pressure of a mixture of gases is equal to the sum of the pressures of all of the constituent gases alone.

Mathematically PTotal = P1 + P2 … Pn

Total pressure of dry air = Pressure of (nitrogen + oxygen + carbon dioxide + inert gases).

The partial pressure of any particular gas in a mixture can be calculated by multiplying the percentage of the gas by the total pressure of the mixture.

For example, the partial pressure of oxygen in dry air at sea level is:

21% x 101(kPa) = 21/100 x 101 = 21.2 kPa