Which statement best describes physiologic dead space?

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Physiologic dead space is the total air in the lungs that does not participate in gas exchange. It is the sum of anatomical dead space (air in the conducting airways) and alveolar dead space (air in alveoli that are ventilated but not perfused). So it represents the portion of each tidal breath that does not reach perfused alveoli to exchange gases. In healthy individuals this dead space is mostly anatomical, but it can increase with processes that reduce perfusion of ventilated alveoli. This is why it’s not describing the portion of tidal volume that does participate in gas exchange—that would be the functional, gas-exchanging portion, i.e., alveolar ventilation when you account for dead space. Alveolar ventilation itself is calculated from tidal volume minus physiologic dead space, times the respiratory rate, not the dead space as a description.