Resp5 - Gas Exchange in the Lungs

Question 1

How to work out Alveolar Gas Pressure starting from atmospheric pressure

Atmospheric Pressure
Partial Pressure of Oxygen
Water Vapour Pressure
Alveolar Gas Pressure
Venous Gas Pressure
Alveolar-arterial Gradient

Answer 1

1. ) Atmospheric Pressure - sum of the partial pressures of the gases in the atmosphere
   - 101.3 kPa = 1 atmospheric pressure at sea level
   - decreases with higher altitude because air molecules become more dispersed

2.) Partial Pressure of Oxygen - oxygen makes up 21% of air so partial pressure of oxygen is 101kPa x 0.21 = 21kPa

3. ) Water Vapour Pressure - inspired air is warmed and humidified in the URT so you subtract the water vapour pressure from the atm pressure
   - 101kPa - 6.3kPa = 94.7kPa is the pressure of inspired and moistened air at sea level
   - therefore p(O2) of inspired air = 94.7 x 0.21 = 20kPa
4. ) Alveolar Gas Pressure - different from atmospheric because of anatomical dead space
   - alveolar gas pressure are equivalent to the arterial gas pressures
   - 30% of tidal volume fills the anatomical dead spaces
   - alveolar pa(O2) is 13.3kPa rather than 20kPa
   - alveolar p(aCO2) is 5.3kPa
   - venous pv(O2) is 5.3kPa, venous pv(CO2) is 6.1kPa

5.) A-a Gradient - alveolar P(O2) - arterial P(O2) [PAO2 - Pa02]
- <2kPA in young, <4 in elderly
- >5kPA suggests lung pathology
- PAO2 = room air (20kPa) - PACO2/0.8
-

Question 2

Calculating amount of a dissolved gas

Formula
Solubility Coefficient
Binding to Haemoglobin

Answer 2

1.) Amount of a gas dissolved (mmol/L) = partial pressure (kPa) x solubility coefficient of gas

2. ) Solubility Coefficient - constant for the individual gas and the solvent (mainly water)
   - solubility coefficient of oxygen in plasma is 0.01 so the amount of dissolved oxygen in plasma is
   - 13kPa x 0.01 = 0.13 mmol/L
3. ) Binding to Haemoglobin - oxygen bound to haemoglobin is not included in the partial pressure of oxygen
   - after Hb is fully saturated, O2 continues to dissolve till equilibrium is reached

Question 3

4 factors affecting rate of diffusion and solubility of carbon dioxide and oxygen

Answer 3

1. ) Partial Pressure Gradient
   - greater gradient of oxygen than carbon dioxide
2. ) Surface Area - of the membrane
   - decreased by removal of a lung or by emphysema
3. ) Diffusion Distance - thickness of membrane + space
   - increases due to oedema or lung fibrosis

4.) Diffusion Coefficient - determine the rate different gases diffuse at the same pressures and membranes

5. ) Carbon Dioxide and Oxygen Solubility
   - CO2 diffuses 20 times faster than oxygen
   - diffusion of CO2 is not affected until late stage disease
   - in lower oxygen levels, oxygen gas exchange is more impaired because of the slower diffusion rate