Wi: Describe the laws governing the behaviour of gases and liquid

Dalton’s Law

  • Definition – in a mixture of gases, the pressure exerted by each gas is the same as the pressure exerted if the gas was the only gas in that mixture

Equation – PTOTAL = PGAS 1 + PGAS 2 + PGAS 3 + … etc

  • Clinical Application (C/A): Alveolar Gas Equation → relevant for partial pressures of gases in alveoli

Boyle’s Law

  • Definition – “for a fixed mass of gas at constant temp, the pressure is inversely proportional to the volume”
Boyle’s Law
  • ↑P = molecules closer together
  • ↑Vol = molecules further apart ∴ ↓P
  • C/A: measuring volume of O2 left in cylinder

Henry’s Law

  • Definition – “the amount of gas dissolved in a liquid is directly proportional to its Partial Pressure @ STP”

Equation: Solubility = Constant x Partial Pressure

  • C/A: Hyperbaric O2 Therapy → @ 1ATM, O2 dissolved is v. small → hyperbaric → ↑P ∴↑ dissolved O2

Graham’s Law

Graham’s Law
  • C/A: Helium → small molecular weight ∴ used to improve diffusion & gas exchange in airway obstruction

Fick’s Law of Diffusion

  • Definition – passive movement of molecules from an area of high concentration to low concentration

The rate of diffusion is


  • SA
  • Solubility
  • Pressure gradient

Inversely Proportional

  • Square root of MW
  • Thickness
  • C/A: Diffusion across blood gas barrier