Partial pressure  In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas 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. The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. Gases dissolve, diffuse, and react according to their partial pressures, and not according to their concentrations in gas mixtures or liquids. This general property of gases is also true in chemical reactions of gases in biology. For example, the necessary amount of oxygen for human respiration, and the amount that is toxic, is set by the partial pressure of oxygen alone. This is true across a very wide range of different concentrations of oxygen present in various inhaled breathing gases or dissolved in blood. The partial pressures of oxygen and carbon dioxide are important parameters in tests of arterial blood gases, but can also be measured in, for example, cerebrospinal fluid. The symbol for pressure is usually or which may use a subscript to identify the pressure, and gas species are also referred to by subscript. When combined these subscripts are applied recursively. Examples:  Dalton's law expresses the fact that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the individual gases in the mixture. This equality arises from the fact that in an ideal gas the molecules are so far apart that they do not interact with each other. Most actual real-world gases come very close to this ideal. For example, given an ideal gas mixture of nitrogen (N2), hydrogen (H2) and ammonia (NH3):  Ideally the ratio of partial pressures equals the ratio of the number of molecules. That is, the mole fraction formula_8 of an individual gas component in an ideal gas mixture can be expressed in terms of the component's partial pressure or the moles of the component:  and the partial pressure of an individual gas component in an ideal gas can be obtained using this expression:  The mole fraction of a gas component in a gas mixture is equal to the volumetric fraction of that component in a gas mixture. The ratio of partial pressures relies on the following isotherm relation:  The partial volume of a particular gas in a mixture is the volume of one component of the gas mixture. It is useful in gas mixtures, e.g. air, to focus on one particular gas component, e.g. oxygen.