A significant fall in pulmonary transit time with prolonged exercise has been seen. Pulmonary vascular bed expands as cardiac output and pulmonary artery pressure increases with exercise, thereby minimizing the reduction in red cell transit time. At rest pulmonary transit time is ~ 0.9 s, while at maximal exercise it is reduced to 0.5 s. But PaO2 is maintained by the increase in alveolar-capillary pressure gradient afforded by the hypercentilation of heavy exercise. Secondly, V/Q is distributed throughout the lungs during exercise.
