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Physiologic Dead Space & Ventilation Rates

Physiologic Dead Space & Ventilation Rates

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Key points:
  • Gas exchange requires the close physical association of ventilated alveoli and perfused pulmonary capillaries.
  • However, in the dead spaces of the respiratory tract, one (or both) of these requirements is absent, and gas exchange does not occur.
Anatomic dead space: conduction portion of the respiratory tract (we show the tracheobronchial tree in this image). Functional dead space: aka, alveolar dead space comprises alveoli where gas exchange does not occur (i.e., non-perfused alveoli). Physiologic dead space: includes the anatomical space and functional dead space; this is the total volume of the respiratory tract that does not participate in gas exchange. It can be calculated using the Bohr equation.
Ventilation
Minute ventilation rate
  • The total rate of air-flow into and out of the lungs.
  • Includes the air-flow through the tracheobronchial tree and to both the functional alveoli and non-functional alveoli.
Alveolar ventilation rate
  • Refers to the rate of air-flow into and out of the functioning alveoli, only (not though the physiologic dead spaces).
Key Points
  • When alveolar ventilation rate decreases, the alveolar partial pressure of carbon dioxide increases while the partial pressure of oxygen decreases.
    • This makes intuitive sense, as you can imagine the effects of reducing alveolar ventilation by holding your breath:
  • Carbon dioxide is held in your lungs, and the amount of oxygen will necessarily decrease because you aren't bringing fresh air into your lungs.
  • More specifically, the alveolar gas equation predicts the degree of change in alveolar partial pressure of oxygen for a given change in the partial pressure of alveolar carbon dioxide, as long as the respiration quotient stays at 0.8:
For example, if alveolar ventilation is reduced by one half, alveolar partial pressure of carbon dioxide increases two-fold, and partial pressure of oxygen decreases by slightly more than one-half.
  • Notice that the alveolar gas equation also informs us that if the respiratory quotient changes, the relationship between alveolar partial pressure of carbon dioxide and oxygen will also change.

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