Diffusing capacity

Master answer

• Diffusion capacity:
  • “The volume of gas which will diffuse through a membrane each minute for a partial pressure difference of 1mmHg (ml/min/mmHg)”
  • Normal diffusion capacity for oxygen (D­L_O2): 20ml/min/mmHg
• Fick equation:

• Thickness of membrane (normal: 0.3μm)
• Surface area of membrane (normal: 50-100m²)
  • Age, posture, degree of inflation (1 mark for any)
• Partial pressure gradient across membrane
  • Rate of blood flow through lungs
  • (no points for mentioning diffusion constant of gas – question relates to only oxygen)
• Rate of oxygenation of reduced Hb
  • Shift of oxygen dissociation curve (pH, temperature, PCO2, 2,3-DPG)
  • Haematocrit
  • Abnormalities of haemoglobin

In exercise

• DL­_O2 increases to 30-60ml/min/mmHg
• Increased VT and RR
  • Increased alveolar ventilation → partial pressure gradient of O2 across capillary increases (significant as flow of erythrocytes through pulmonary vasculature likely to increase)
• Recruitment of dormant pulmonary capillaries (increased surface area for diffusion)
  • due to: Increased lung inflation, Increased pulmonary arterial systolic pressure­­­­­­­
  • Perfusion increases from 21ml/min/mmHg up to 65ml/min/mmHg
• Better matching of ventilation-perfusion

Mooney 2016

Answer by Mooney