ABSTRACT. The difference between the alveolar and arterial oxygen pressure is known as alveolar – arterial difference (PA-aO2) or efficiency for gas exchange. Gas exchange within the lung is not perfect even at rest. During exercise, the gas exchange progressively worsens. The mechanisms that contribute to A-a DO2 are not well known. Ventilation-perfusion mismatch (VA/Q) is considered to be the main reason for A-aDO2 widening. VA/Q mismatch is due to the effect of gravity - structural differences in airways – blood vessels, bronchoconstriction, secretions from airways irritated by high flows or dry-cold air and mild interstitial edema. Diffusion limitation is a second contributing factor to A-aDO2 widening. It can be attributed to several reasons, such as: the surface area for diffusion, the distance required for diffusion from the alveolar membrane to the red blood cell, the transit time and the rate of equilibration of mixed venous blood with alveolar gas. Diffusion limitation may occur to well-trained athletes at high exercise intensity, while it is unlikely to occur in untrained subjects during low and moderate exercise. The last contributing factor to A-aDO2 widening is considered to be the mixing of shunted blood (intra-extrapulmonary shunt) with arterial blood. Even though there is not enough published data to support the previous statement, it seems that not only the A-aDO2 is negatively affected but simultaneously may play a positive role by protecting the alveolar – capillary membrane from liquid accumulation and disruption in athletes. Pneumon 2004, 17(3):265-271.