Partial Pressures

Unlike liquids, gases expand to fill the volume available to them, and the volume occupied by a given number of gas molecules at a given temperature and pressure is (ideally) the same regardless of the composition of the gas. Therefore, the pressure exerted by any one gas in a mixture of gases (its partial pressure) is equal to the total pressure times the fraction of the total amount of gas it represents.

The composition of dry air is 20.98% O₂, 0.04% CO₂, 78.06% N₂, and 0.92% other inert constituents such as argon and helium. The barometric pressure (P_B) at sea level is 760 mm Hg (1 atmosphere). The partial pressure (indicated by the symbol P) of O₂ in dry air is therefore 0.21 × 760, or 160 mm Hg at sea level. The partial pressure of N₂ and the other inert gases is 0.79 × 760, or 600 mm Hg; and the PCO₂ is 0.0004 × 760, or 0.3 mm Hg. The water vapor in the air in most climates reduces these percentages, and therefore the partial pressures, to a slight degree. Air equilibrated with water is saturated with water vapor, and inspired air is saturated by the time it reaches the lungs. The PH₂O at body temperature (37 °C) is 47 mm Hg. Therefore, the partial pressures at sea level of the other gases in the air reaching the lungs are PO₂, 149 mm Hg; PCO₂, 0.3 mm Hg; and PN₂ (including the other inert gases), 564 mm Hg.

Gas diffuses from areas of high pressure to areas of low pressure, the rate of diffusion depending upon the concentration gradient and the nature of the barrier between the two areas. When a mixture of gases is in contact with and permitted to equilibrate with a liquid, each gas in the mixture dissolves in the liquid to an extent determined by its partial pressure and its solubility in the fluid. The partial pressure of a gas in a liquid is that pressure which in the gaseous phase in equilibrium with the liquid would produce the concentration of gas molecules found in the liquid.