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”
- ↑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
- 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
Proportional
- SA
- Solubility
- Pressure gradient
Inversely Proportional
- Square root of MW
- Thickness
- C/A: Diffusion across blood gas barrier