Anaes · Measurement & monitoring physics

The gas laws

Also known as Gas laws · Boyle's law · Charles's law · Ideal gas equation · Dalton's law · Henry's law

Anaesthesia is the science of gases under pressure — the contents of a cylinder, the volume in a breathing system, the partial pressures that drive uptake and diffusion, and the physics of altitude and hyperbaric therapy all obey the gas laws. The framework rests on six exam-critical ideas. First, the ideal-gas assumption: a gas is a cloud of point molecules in random motion exerting a pressure by collision with the walls, and its state is summarised by the four variables pressure (P), volume (V), temperature (T) and amount (n). Second, BOYLE'S LAW holds that at constant temperature the pressure of a fixed mass of gas is inversely proportional to its volume (P times V is constant) — the basis of how a gas is compressed into a cylinder and how a squeeze-bag ventilator works. Third, CHARLES'S LAW holds that at constant pressure the volume is proportional to the absolute temperature (V divided by T constant), and GAY-LUSSAC'S LAW that at constant volume the pressure is proportional to the absolute temperature — together explaining why a gas warms as it is compressed and cools as it expands. Fourth, AVOGADRO'S HYPOTHESIS — equal volumes of different gases at the same temperature and pressure contain equal numbers of molecules — links the amount of gas to its volume and defines the mole and the molar volume at STP (22.4 litres per mole). Fifth, these combine into the UNIVERSAL (IDEAL) GAS EQUATION, PV equals nRT (where R is the universal gas constant, 8.314 joules per mole per kelvin), from which any one variable can be found given the other three. Sixth, two mixture laws govern the behaviour of gas mixtures: DALTON'S LAW, that the total pressure of a gas mixture is the sum of the partial pressures of its individual gases (so the partial pressure of oxygen in air is 21 percent of the total), and HENRY'S LAW, that the amount of a gas dissolved in a liquid is proportional to its partial pressure — the basis of oxygen and anaesthetic uptake by the blood and of decompression sickness. Real gases deviate from ideal behaviour at high pressure or low temperature; nitrous oxide, whose critical temperature is above room temperature, liquefies in the cylinder and so shows a constant pressure (its saturated vapour pressure) until the liquid is exhausted, unlike oxygen which remains gaseous and shows a pressure proportional to its contents. Built on the partial-pressure-of-oxygen primer (Hoecker 2026), the hyperbaric-physics review (Jones 2026), the hypobaric-oxygenator physics study (Chotimol 2025), the nitrous-oxide-piped-supply-loss report (Hirata 2026), the greener-nitrous-oxide report (Yadav 2026), the chronic-hypoxaemia study (Ricco 2026), the decompression-sickness-at-altitude report (Ben-Ari 2026), and the pulse-oximetry review (Moon 2026).