5 - Partial Pressure and Diffusion Flashcards
What is Boyle’s law and what can it be applied to in respiratory?
- Pressure (KPa) of a fixed quantity of gas at a constant temperature is inversely proportional to it’s volume
- P = nR.T/V
What is Dalton’s Law?
Each gas exerts a partial pressure proportional to it’s volume. e.g 20.9% oxygen
What is atmospheric pressure and how much of this is oxygen?
- 101 KPa at sea level, decreases as altitude increases
- 20% oxygen
What is the pO2 and pCO2 in arteries, veins, alveoli and conducting airways?
Why is the partial pressure of oxygen lower in the alveoli than in the conducting airways?
Because when air enters the airways it becomes warmed and moistened so water vapour is added, which is 6.28KPa, need to subtract this from 101KPa
What is the relation of gases in liquid and gases in gas (e.g at the alveolar fluid interface)
At equilibrium the partial pressure of the gas in the liquid is equal to the partial pressure of the gas in the gas phase in contact with the liquid
What is the difference between partial pressure of gas and the amount of gas dissolved?
Amount of gas dissolved (mmol/L): Partial pressure (KPa) x Solubility coefficient of gas (depends on temp)
(Henry’s Law)
What is the amount of oxygen dissolved in plasma and how is the demand for oxygen in the body achieved?
- Amount dissolved = 0.01mmol/L/KPa x 13.3 KPa = 0.13 mmol/l
- O2 dissolves into blood and associates with Hb. When fully saturated 8.8mmol/L
- Total O2 = 8.8 (99%) + 0.13 (1%) = 8.93mmol/L
What is saturated vapour pressure?
At equilibrium the gas mixture is saturated with water vapour that exerts a pressure of 6.28kPa at 37 degrees
What is pO2 and pN2 of humidified air?
pATM = 101
SVP = 6.28 kPa
pO2 = (101-6.28) x 0.2 = 19.8KPa
pN2 is 78% of air
What is alveolar pCO2 and pO2 determined by?
Rate at which O2 is taken up by the blood and the **rate it is replenished by **ventilation. Opposite for CO2
Because of pressure gradients CO2 moves out and O2 in
What factors affect the rate of diffusion in the lungs?
- Surface area (70m2)
- Gradient of partial pressure
- Thickness (0.6 micron)
- Solubility of gas
- MWt (less so than above)
What are the layers that gases have to pass through from the alveoli to the blood?
In lung disease where there is a diffusion defect, e.g emphysema, what happens to the arterial pO2 and pCO2?
- pCO2 not usually affected as it diffuses 21 times faster than O2 as more soluble in water. Only falls in late stage of disease
- pO2 will fall
- Need to keep alveolar pressure relatively stable to perfuse the blood with what it requires in the half a second. Done by ventilation
What does hypo and hyperventilation do to pCO2 and pO2?
What are some diseases that cause diffusion defects in the lungs?
Fibrotic Lung Disease : Thickened alveolar membrane slows gas exchange
Emphysema : Destruction of alveoli reduces surface area
Pulmonary Oedema : Fluid in interstitial space increases diffusion distance
How do you measure diffusion resistance in the lungs?
Why do you get decompression sickness from diving?
Double the atmospheric pressure so double the amount of nitrogen that dissolves in the blood so when you come back up to normal pressure the nitrogen goes back to gas and forms painful bubbles
What will happen to the pO2 of somebody climbing Mt Everest?
Atmospheric pressure drops so pO2 of oxygen will fall, may not be able to perfuse
