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.
The partial pressure of carbon dioxide in the alveoli is proportional to the fraction of carbon dioxide () multipled by , a constant.
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.
The rate of transfer of a gas through a sheet of tissue is proportional to the tissue area and the difference in gas partial pressures between the two sides, and inversely proportional to tissue thickness.
Where is the diffusion constant (Graham’s law), stating that diffusion is proportional to solubility but inversely proportional to the square root of the molecular weight (or density).
Carbon dioxide diffuses around twenty times more rapidly than oxygen does due to increased solubility despite a higher molecular weight.
The rate of diffusion through a tissue is proportional to the cross sectional area but inversely proportional to the thickness.
The rate of diffusion is proportional to the pressure differential between either side of the tissue.
The more soluble a substance is the more rapidly it will diffuse, but the larger it’s molecular weight the slower it will diffuse.