GASEOUS EXCHANGE & FACTORS DETERMINING

• Diffusion of gases from alveoli to blood (O2) or from blood to alveoli (CO2), happens via alveolocapillary (respiratory) membrane.
• This is gaseous exchange process.

Factors affecting gasous diffusion:

1. Thickness of respiratory membrane: 

• Normal thickness – 0.5 µm,
• Thickening of membrane seen in:
  • Pneumonia, TB, Lung carcinoma, Emphysema & Cystic fibrosis.

2. Surface area of membrane:

• Total surface area in two lungs = 100 m².
• Time taken by blood to traverse pulmonary capillaries at rest: 0.75 s.

3. Diffusion coefficient of gas:

• Diffusion – Net movement of fluid from one compartment to another due to concentration difference.
  • Gas diffuses from high to low pressure.
• Diffusion rate (D) is directly proportionol to,
  • Pressure gradient (P).
  • Solubility of gas (S).
  • Surface area of respiratory membrane (A).
• Diffusion rate (D) is inversely proportionol to,
  • Thickness of respiratory membrane (d).
  • Molecular weight of gas (MW).
• Diffusion coefficient – S/√ MS.
  • Main characteristics determining rate of diffusion of gas – S & MW.
• Diffusion coefficient of oxygen = 1
• Diffusion coefficient of carbon-dioxide = 20.3.
  • Diffusion coeficient of CO2 > O2 – Due to more solubility of CO2.
  • CO2 diffuses more rapidly into respiratory membrane than O2.

4. Solubility coefficient:

• Denotes the solubility characteristics of particular gas in water.
• Solubility coefficiency for oxygen – 0.024
• Solubility coefficiency for CO2 – 0.57
  • CO2 is 20 times more soluble than O2.
• Solubility coefficiency for N2 – 0.012
• Solubility coefficiency for CO – 0.018

5. Diffusion capacity:

• Is the overall ability of respiratory membrane to transfer a gas between alveolar air & pulmonary blood.
• Also referred as “transfer factor”.

• Diffusion capacity of O2 – 25 ml/min/mm Hg (At rest).
• Diffusion capacity of O2 – 65 ml/min/mm Hg (At exercise).

• Diffusion capacity of CO2 (theoretical value) – 400 ml/min/mm Hg (At rest).

• Diffusion capacity of CO – Used as index for measuring normalcy.
  • (Note: Diffusion of Co & O2 are same).

Factors affecting diffusion capacity of CO:

• Decreasing factors:
  • Restrictive/interstitial lung disease, COPD (emphysema), pulmonary embolism, pulmonary hypertension, CHF, anemia & drugs (amiodarone).
• Increasing factors:
  • Polycythemia, asthma, left to right shunt, alveolar hemorrhage, increased pulmonary blood volume (during exercise).