Alveolarventilation

Alveolar ventilation is the portion of tidal ventilation that reaches the gas-exchanging units of the lungs—the alveoli—and participates in the exchange of oxygen and carbon dioxide. It reflects how effectively air reaches functionally perfused alveoli, rather than merely moving in and out of the airways.

Alveolar ventilation is influenced by three main factors: tidal volume (the amount of air moved with each breath), respiratory rate (breaths per minute), and dead space (air that does not participate in gas exchange). Dead space includes anatomical structures in the conducting airways and any alveoli that are ventilated but not perfused or poorly perfused. The concept is distinct from minute ventilation, which equals tidal volume times respiratory rate and includes the air wasted in dead space.

The standard relationship is VA = (VT − VD) × f, where VA is alveolar ventilation, VT is tidal volume, VD is dead space volume, and f is the respiratory rate. Estimations often use approximate values: in a healthy adult at rest, VT about 0.5 L, VD about 0.15 L, and f about 12 breaths per minute, giving VA around 4.2 L/min.

Clinical relevance lies in how VA affects gas exchange and acid–base status. PaCO2 is largely determined by the balance between CO2 production and alveolar ventilation (PaCO2 ∝ VCO2/VA). Increases in VA lower PaCO2, while reductions in VA raise it. Conditions such as COPD, pulmonary embolism, or anesthesia can alter VA by changing VT, f, or VD, affecting oxygen delivery and carbon dioxide elimination.