From a pathophysiologic perspective, the changes that occur in asthma are multiple, diverse, and complex. Assessment of the mechanical properties of the ventilatory apparatus provides several different types of information, depending on the gravity of the bronchial obstruction. During asthma, or induced bronchial obstruction, the function of the muscles is altered, causing changes in respiratory timing. Expiratory duration decreases more than inspiratory duration, and the functional residual capacity (FRC) increases, due to mechanical changes within the airways that lead to air trapping. The related hypoventilation is responsible for hypoxemia and hypercapnia, but it does not severely affect the diffusion capacity of the alveolocapillary membrane. We describe the pathophysiology of the bronchial obstruction in asthmatic patients, underlining the critical function of the respiratory muscles. Moreover, we clarify the relations between the ventilatory changes and gas-exchange alteration.