ventilation:perfusion ratio

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ven·ti·la·tion:per·fu·sion ra·ti·o

(V̇V˙A:Q˙) (ven'ti-lā'shŭn-pĕr-fyū'zhŭn rā'shē-ō)
The ratio of alveolar ventilation to simultaneous alveolar capillary blood flow in any part of the lung; because both ventilation and perfusion are expressed per unit volume of tissue and per unit of time, which cancel, the units become liters of gas per liter of blood.