Partial Pressure and Diffusion Flashcards
What is Dalton’s Law?
The total pressure of gases= the sum of partial pressure of individual gases
What is atmospheric pressure at sea level?
101 kPa
What is the composition of ambient air?
- 79% Nitrogen
- 21% Oxygen
- 0.04% CO2
How do you calculate the partial pressure of oxygen at sea level?
Work out the fraction of oxygen in air x atmospheric pressure
21% x 101 kPa = 21kPa
What is important to remember about inspired air and how does this effect the pO2 in the upper respiratory tract?
Inspired air is warmed and humidifed
therefore… calculating pO2 you must subtract water vapour pressure (6.3 kPa)
101-6.3 kPa = 94.7 kPa
pO2 = 0.21 x 94.7 kPa= 20 kPa
Why is alveolar partial pressure lower than partial pressure in the upper respiratory tract?
In the alveoli, O2 constantly diffuses to the capillaries
Alveolar pO2 is determined by the rate at which O2 is taken up by blood
Anatomical dead space means fresh air is also diluted by old air
Fill in the table
What happens when gas mixtures come in contact with a liquid?
- Gas molecules enter liquid and dissolve
- Some dissolved molecules return to gas phase
What is the partial pressure of gas in liquid at equilibrium?
Partial pressure of gas in the liquid is equal to partial pressure of the gas in the gas phase in contact with the liquid
Is pO2 the same as the amount of a dissolved in gas?
No
Amount of dissolved gas = partial pressure x solubulity coefficient
How does O2 bidning to Hb affect pO2?
Once O2 binds to Hb it no longer contributes to partial pressure of blood
Binding to Hb maintains a strong gradient for O2 to dissolve into plasma
pO2 is a measure of dissolved blood only
Proportionally, how much O2 is dissolved in plasma vs bound to Hb?
98-99% to Hb
1-2% dissolved
What is mixed venous blood?
A mix of oxygenated and deoxygenated blood returning to the IVC
What are the pO2 and pCO2 in mixed venous blood?
How does this affect gas movement at alveoli?
pO2 = ~6.0 kPa
pCO2 = ~6.1 kPa
What factors affect the rate of diffusion?
- Area available for exchange
- Distance to diffuse
- Gradient of partial pressure
What properties of a gas affect diffusion?
- Solubility
- Molecular weight (higher weight= slower diffusion)
Out of CO2 and O2, which diffuses faster and why?
CO2 diffuses faster
- CO2 has a larger MW but is much more soluble than O2
- Effect of solubulity much greater so CO2 diffuses 20x faster than O2
What happens to levels of pCO2 and pO2 in blood, during hypoventilation?
pCO2 goes up as not breathed out properly
pO2 goes down
What diffusion barriers exists between alevolar air and the RBC?
- Alveolar epithelial cells
- Interstitial fluid
- Capillary endothelial cells
- Plasma
- RBC membrane
In a healthy lung, how long does it take for O2 exchange to complete?
in 1/3 of the time blood spends in lung capillary bed
How can disease affect the thickness of the membrane for diffusion?
- oedema in interstitial space and alveoli increases thickness
- lung fibrosis thickens alveolar & capillary membrane and interstitium
How can disease affect S.A of the membrane to affect gas diffusion?
- Removal of entire lung
- Emphysma reduced S.A
What happens to pO2 at high altitude?
Decreases
Climbers may need O2
What happens to pO2 under water? Therefore what pressure is air in a scuba tank?
increases
pressure= atmospheric pressure + weight of water
Scuba tank air is much higher than dry land
