Diffusion

Physics of gas diffusion and partial pressures • Diffusion- free & random movement of molecules in liquid or gaseous state, except at absolute zero temperature

• Partial pressures of individual gases is directly proportional to concentration of gas molecules • Atmospheric air- O2 conc is 21% and partial pressure(PO2) is 160mm Hg. N2 conc is 79% and PN2 is 600m Hg • Factors determining partial pressures of gases in liquid: (i) concentration of gas (ii) solubility coefficient

conc of dissolved gas Partial pressure=------------------------------solubility coefficient Solubility coefficients of important gases: Oxygen-0.024 Carbon dioxide– 0.57 Carbon monoxide– 0.018 Nitrogen-0.012 Helium-0.008

Factors determining net rate of diffusion • D α ΔPxAxS dx√MW Where D= net diffusion rate ΔP= partial pressure difference A= cross-sectional area S= solubility coefficient d= distance/thickness MW= molecular weight

Diffusion coefficient: proportional to S/√MW Relative diffusion coefficients: Oxygen 1.0 Carbon dioxide 20.3 Carbon monoxide 0.81 Nitrogen 0.53 Helium 0.95

Composition of alveolar air • Humidification occurs during inspiration • PH2O is 47mm Hg at body temperature

Atmospheric air (mm Hg)

Humidified Alveolar air air (mm Hg) (mm Hg)

Expired air

N2

597.0 (78.62%)

563.4 (74.09%)

569.0 (74.9%)

566.0 (74.5%)

O2

159.0 (20.84%)

149.3 (19.67%)

104.0 (13.6%)

120.0 (15.7%)

CO2

0.3 (0.04%)

0.3 (0.04%)

40.0 (5.3%)

27.0 (3.6%)

H2O

3.7 (0.50%)

47.0 (6.20%)

47.0 (6.20%)

47.0 (6.20%)

(mm Hg)

Rate of renewal of alveolar air

• FRC- 2300ml • Tidal alveolar ventilatory volume- 350ml • Importance of slow replacement(i) makes respiratory control mechanism more stable (ii) avoids sudden changes in PO2, PCO2 and Ph when respiration is temporarily stopped.

[O2] & PO2 in alveoli: controlled by (i) rate of alveolar ventilation and (ii) rate of entry of new oxygen by ventilation [CO2] & PCO2 in alveoli: directly proportional to rate of CO2 excretion and inversely proportional to ventilatory rate Expired air:

Respiratory membrane • Alveolo-capillary membrane/pulmonary membrane • 0.3μ to 1.0μ thickness • Layers- (i) fluid lining alveolus (ii) alveolar epithelium (iii) epithelial basement membrane (iv) interstitial space (v) capillary basement membrane (vi) capillary endothelium

Factors affecting gas diffusion through respiratory membrane • • • •

Thickness of membrane Surface area of the membrane Diffusion coefficient of gas Pressure difference of gas

Diffusing capacity of respiratory membrane • Def- volume of gas that diffuses each minute across the membrane under a pressure difference of 1mm Hg • Diffusing capacity of O2- 21ml/min/mm Hg at rest and 65ml/min/mm Hg during exercise • Diffusing capacity of CO2 is 400ml at rest & 1200ml/min/mm Hg during exercise • Diffusing capacity is measured using CO.

Ventilation-perfusion ratio • Balancing between ventilation & perfusion • Resting ventilation (VA): 4l/min • Resting pulmonary perfusion (Q): 5l/min • Resting VA/Q: 0.8 • When VA/Q=0, there is no ventilation • When VA/Q=∞, there is no perfusion

Physiological shunt • VA/Q is below normal • Total amount of blood shunted per minute is called physiological shunt

Physiological dead space • VA/Q is above normal • Wasted work of ventilation • Calculated by bohr’s equation

Abnormalities of VA/Q • In upper lung, it is 2.5 times the normalphysiological dead space • In the lower lung it is 0.6 times the normalphysiological shunt • In COPD, some areas of lung may exhibit physiological shunt and some areas physiological dead space