The normal dead space is: (DD8)
|A||30% of tidal ventilation|
|B||50% of tidal ventilation|
|C||57% of tidal ventilation|
|D||60% of tidal ventilation|
30% of tidal ventilation
Dead space is constituted by the air which does not participate in diffusion.
Total dead space (physiological dead space) = anatomical dead space (constituted by air present in airways) + alveolar dead space (V/Q mismatch).
Anatomical dead space is 150 ml or 2 ml/kg or 30% of tidal volume.
Alveolar dead space is 60-80 ml in standing and nil in lying (in lying perfusion is same in all parts of lung).