Dead space

The volume within the respiratory system that does not participate in gas exchange.

Anatomical dead space
The volume of the conducting airways (~150mls)

Physiological dead space
Volume of alveoli that do not eliminate carbon dioxide, plus the anatomical dead space volume


In health the anatomical dead space volume is the same as the physiological dead space volume, however in lung disease and other clinical situations the physiological dead space may get larger.

Measuring alveolar ventilation

Because no gas exchange occurs in the anatomical dead space all carbon dioxide in expired gases must come from alveolar gas.

VCO2 = VA  x  FCO2

Therefore VA  =   frac{VCO2}{FCO2}

The partial pressure of carbon dioxide in the alveoli is proportional to the fraction of carbon dioxide (FCO2) multipled by K, a constant.

VA  =  frac{VCO2}{PCO2}  x  K

This is the alveolar ventilation equation. The alveolar ventilation is inversely proportional to the partial pressure of carbon dioxide.

The Bohr equation is used to calculate the fraction of dead space in the lungs.


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