Gas Exchange: Properties Of Gases And Diffusion Flashcards
What is Boyle’s law?
Pressure of a gas is inversely proportional to its volume
How do you calculate the partial pressure of a gas in moist air?
Use O2 as an example
pX = (atm - SVP) x X
Atmospheric pressure: 101kPa
Saturated vapour pressure: 6.28kPa
O2 makes up 20.9% of air
pO2 = (101 - 6.28) x 0.209 = 19.8kPa
How do you calculate the amount of a dissolved gas?
partial pressure x solubility coefficient of gas
What is the total pressure of a mixture of gases?
The sum of the partial pressure of all the gases
Gases dissolve + diffuse according to their…
Partial pressures
From area of high partial pressure to lower partial pressure
The amount of dissolved gas in a liquid is proportional to its ** above the liquid and to the ** in that liquid
The amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid and to the gas’ solubility in that liquid
What is the amount of dissolved gas in a liquid proportional to?
- it’s partial pressure above the liquid
- the gas’ solubility in the liquid
What layers do gas have to diffuse through from alveolar air to RBC?
- fluid film lining alveolus
- epithelial cell of alveolus
- interstitial space
- endothelial cell of capillary
- plasma
- red cell membrane
Compare CO2 and O2 in terms of their:
- solubility
- molecular weight
- CO2 is more soluble than O2
- CO2 greater molecular weight
Effect of solubility is greater than MW
How is diffusion impaired in fibrotic lung disease?
Thickened alveolar membrane > larger diffusion distance
How is diffusion impaired in pulmonary oedema?
Fluid in interstitial space > increases diffusion distance
How is diffusion impaired in emphysema?
Destruction of alveoli > reduces SA for gas exchange
What factors affect the rate of diffusion?
- partial pressure difference/gradient
- surface area available for diffusion
- diffusion distance
- diffusion coefficient of the gas
- solubility of the gas in liquid
- molecular weight of gas
How does the solubility of the gas affect diffusion?
Greater the solubility
Faster the rate of diffusion
How does the molecular weight of a gas affect diffusion?
Higher molecular weight
Slower rate of diffusion
Normal ventilation perfusion ratio
0.8
What does V + Q stand for in V/Q ratio?
V - alveolar ventilation
Q - perfusion (pulmonary blood flow)
What does normal V/Q depend on?
Normal resp rate
Tidal volume
Cardiac output
What is the ideal V/Q?
1
Ventilation = perfusion
Typical V/Q ratio across the lungs
- alevoli at apex: under-perfused + over-ventilated > high V/Q ratio
- alveoli at base: over-perfused + under-ventilated > low V/Q ratio
Examples of conditions where there’s a V/Q mismatch of <1
Conditions where there is poor ventilation to the alevoli
pneumonia, moderate asthma, COPD
Examples of conditions where there’s a V/Q mismatch of >1
Conditions where there is poor perfusion to the alevoli
pulmonary embolism
What is the V/Q ratio if there is inadequate ventilation?
V/Q < 1
What is the V/Q ratio if there is inadequate perfusion?
V/Q > 1
