GAS LAWS AND THEIR IMPORTANCE IN ANESTHESIA

➡️Boyle’s law

🔻At a constant temperature, the volume of a fixed amount of a perfect gas varies inversely with its pressure.

🔻PV = K or V ∝ 1/P . Also P 1 V 1 = P 2 V 2

Ⓜ️NEMO> water Boyle’s at a constant temperature

🔻P 1 V 1 relates to the pressure and volume in the cylinder and P 2 V 2 relates to the pressure and volume at atmospheric pressure. 

🔻For example, oxygen is stored at 13800 kPa (absolute pressure) in gas cylinders. If the internal volume of the cylinder is 10 litres, the volume this cylinder will provide at atmospheric pressure: 13800 × 10 = 100 × V2. So V2 = 1380 litres. However, 10 litres will remain within the cylinder, so 1370 litres will be usable at atmospheric pressure.

➡️Charles’ law

🔻At a constant pressure, the volume of a fixed amount of a perfect gas varies in proportion to its absolute temperature.

🔻V/T = K or V ∝ T

Ⓜ️NEMO> Prince Charles is under constant pressure to be king

➡️Gay–Lussac’s law (The third gas law)

🔻At a constant volume, the pressure of a fixed amount of a perfect gas varies in proportion to its absolute temperature.

🔻P/T = K or P∝T 

🔻Perfect gas: A gas that completely obeys all three gas laws or A gas that contains molecules of infinitely small size, which, therefore, occupy no volume themselves, and which have no force of attraction between them.

🔻It is important to realize that this is a theoretical concept and no such gas actually exists. Hydrogen comes the closest to being a perfect gas as it has the lowest molecular weight. In practice, most commonly used anaesthetic gases obey the gas laws reasonably well.

Other gas laws of relevance:

➡️Avogadro’s hypothesis: at a constant temperature and pressure, all gases of the same volume contain an equal number of molecules.

➡️Dalton’s law: the pressure exerted by a mixture of gases is the sum of the partial pressures of its constituents.

➡️Henry’s law: at a constant temperature, the amount of gas dissolved in a given volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with the liquid.

🔻Henry’s law can be used to show that the amount of oxygen dissolved in blood is proportional to the partial pressure of oxygen in the alveolus. The amount of dissolved oxygen carried in blood is 0.023 ml dl−1 kPa−1 . At atmospheric pressure, this accounts for a very small and insignificant fraction of oxygen delivery. However, under hyperbaric conditions, the dissolved fraction increases and becomes a more significant source of oxygen delivery to tissues