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
What happens in V/Q mismatch if there is inadequate ventilation?
- imparied gas exchange in affected alevoli
- pO2 falls
- pCO2 rises
- lung hypoxic vasoconstriction occurs
- causes diversion of blood to better ventilated parts
- Hb in well ventilated alveolar capillaries are saturated already
- no more O2 can bind
- pO2 remains low
- hyperventilation occurs > normal/low pCO2
What is a respiratory shunt?
When you perfuse an unventilated alveolus, that bit of blood is wasted as no gas exchange occurs
What is the compensatory mechanism in VQ mismatch?
Lung hypoxic vasoconstriction
Causes diversion of blood
What is the value of saturated vapour pressure at body temp?
6.28kPa
