Also called pulmonary membrane or alveolocapillary membrane. Its the tissues that separate the capillary blood from alveolar air. Exchange of gases takes place through this membrane and hence it plays a very important role in the respiratory process.
STRUCTURE –> Consists of 8 layers
CHARACTERISTIC FEATURES OF RESPIRATORY MEMBRANE –> Thickness (0.6 micrometre) & Surface area ( 70 square metres) is very crucial for the optimum gas exchange to take place.
FACTORS AFFECTING DIFFUSION ACROSS RESPIRATORY MEMBRANE
DIFFUSION & EQUILIBRIUM OF GASES ACROSS THE RESPIRATORY MEMBRANE
Transit time for blood is ~ 0.75 sec (time blood remains in the lungs capillary).
EQUILIBRIUM TIME –> Normally ~0.25 sec(1/3rd of total transit time) is required for Oxygen as well as Carbon Dioxide diffusion. Rest 0.50 sec is the safety margin (which helps in diffusion during exercise, high altitude or impaired diffusion)
PERFUSION LIMITED vs DIFFUSION LIMITED
Diffusion Capacity of Lung –> It is the quantitative expression of ability of respiratory membrane to exchange gas between alveoli and blood. Volume of gas that diffuses through respiratory membrane of lung each minute for a pressure gradient of 1 mm Hg.
Factors affecting diffusion capacity –>
- Thickness of respiratory membrane (Inversely proportional)
- Surface area of respiratory membrane (Directly proportional)
- Diffusion coefficient (Directly proportional)
- Pressure gradient between the partial pressure of gas in alveolar air and capillary blood (Directly proportional)