Heinonen I, Savolainen AM, Han C, Kemppainen J, Oikonen V, Luotolahti M, Duncker DJ, Merkus D, Knuuti J, Kalliokoski KK. Pulmonary blood flow and its distribution in highly trained endurance athletes and healthy control subjects. J Appl Physiol 114: 329-334, 2013. First published December 6, 2012; doi:10.1152/japplphysiol.00710.2012.-Pulmonary blood flow (PBF) is an important determinant of endurance sports performance, yet studies investigating adaptations of the pulmonary circulation in athletes are scarce. In the present study, we investigated PBF, its distribution, and heterogeneity at baseline and during intravenous systemic adenosine infusion in 10 highly trained male endurance athletes and 10 untrained but fit healthy controls, using positron emission tomography and [ O-15] water at rest and during adenosine infusion at supine body posture. Our results indicate that PBF at rest and during adenosine stimulation was similar in both groups (213 +/- 55 and 563 +/- 138 ml.100 ml(-1).min(-1) in athletes and 206 +/- 83 and 473 +/- 212 ml.100 ml(-1).min(-1) in controls, respectively). Although the PBF response to adenosine was thus unchanged in athletes, overall PBF heterogeneity was reduced from rest to adenosine infusion (from 84 +/- 18 to 70 +/- 19%, P < 0.05), while remaining unchanged in healthy controls (77 +/- 16 to 85 +/- 33%, P = 0.4). Additionally, there was a marked gravitational influence on general PBF distribution so that clear dorsal dominance was observed both at rest and during adenosine infusion, but training status did not have an effect on this distribution. Regional blood flow heterogeneity was markedly lower in the high-perfusion dorsal areas, both at rest and during adenosine, in all subjects, but flow heterogeneity in dorsal area tended to further decrease in response to adenosine in athletes. In conclusion, reduced blood flow heterogeneity in response to adenosine in endurance athletes may be a reflection of capillary reserve, which is more extensively recruitable in athletes than in matched healthy control subjects.