The ventilation-perfusion (V/Q) ratio is the quotient of alveolar ventilation rate — tidal volume minus dead space, multiplied by respiratory frequency — over pulmonary perfusion, defined as cardiac output through the pulmonary capillaries; in a healthy adult this ratio approximates 0.8. Local autoregulatory mechanisms continuously match ventilation to perfusion: alveolar oxygen partial pressure regulates vascular caliber through endothelial nitric oxide signaling (hypoxic pulmonary vasoconstriction, the inverse of systemic oxygen autoregulation), while alveolar carbon dioxide partial pressure regulates bronchiolar smooth muscle caliber, with airflow and blood flow each governed by the flow = pressure gradient / resistance relation and resistance's inverse dependence on diameter. The concept belongs to respiratory physiology within human physiology, specifically the external respiration stage of gas exchange, and connects to cardiovascular physiology through cardiac output and to gas-law theory through Dalton's law of partial pressures.
The ventilation-perfusion (V/Q) ratio is the quotient of alveolar ventilation rate — tidal volume minus dead space, multiplied by respiratory frequency — over pulmonary perfusion, defined as cardiac …