Equations and Laws
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Law of Laplace and what it relates to:
- Law of LaPlace states that pressure in the alveolus is directly proportional to surface tension and inversely to the radius of the alveoli. Thus, pressure in smaller alveoli would be greater thatn in larger alveoli, if
- surface tension were the same in both.
Describe Boyles Law
- Robert Boyle made the discovery that the volume of a gas decreased as the pressure was raised.
- Implies that changes in the intrapulmonary pressure occurs as a result of changes in lung volume.
- “For a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume”
- P is proportional to 1 / V
- PV = K (a constant)
Example: as volume of the lungs increase the pressure of the lungs will decrease during inhalation. When the volume of the lungs decrease then the pressure of the lungs will increase during exhalation.
Describe Charles Law
Charles found that for a fixed mass of gas under constant pressure the temperature of the gas varied directly with the volume.
“For a fixed mass of gas at a constant pressure, the volume is directly proportional to the temperature”
- V is proportional to T
- V / T = K
Describe the pressure law
“For a fixed mass of gas at a constant volume, the pressure is directly proportional to the temperature”
- P is proportional to T
- P / T = K
The Universal Gas Law
- PV = nRT
- n = the number of moles of the gas
- R = the universal gas constant ( 8.31 J K-1)
Only an ideal gas obeys this and previous gas laws exactly and such a gas doesn’t exist! Real gases however, follow the law in our practice as they are at low pressures, and well above their liquefaction temperatures.
Dalton’s Law of partial pressure
John Dalton observed that the total pressure of a gas mixture was the sum of the pressures of each of the gases if they were to exist on their own.
Therefore P mixture = P1+P2+P3 + …
This means that to calculate the total pressure in a cylinder for a mixture of gases just add up all the partial pressures
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