Gravity has shaped the anatomy and physiology of human beings over millions of years. Exposure to microgravity has been shown to affect every single body system. These physiological changes may lead to undesirable health consequences. This paper aims to present and discuss the effects of short and long-term microgravity exposure on lung volumes, capacities and function, along with a final consideration regarding a new method to evaluate respiratory function in space. Parabolic flights have shown that the sternum is displaced in the cranial direction in microgravity and it is accompanied by an increase in diameter of the lower rib cage. This change in the position of the chest wall was predicted to cause the volume-pressure curve to lie between the standing upright and the supine position curves, with a net result of a reduction in lung volumes. In five subjects studied in a KC-135 aircraft, functional residual capacity decreased by 432 ml during microgravity. Vital capacity also reduced from a mean value of 4.72 L at 1 G to 4.35 L at 0G. Forced vital capacity and forced expiratory volume in 1s were also decreased by an average of 2.5% in the 20s of microgravity in a parabolic flight (1). During the 9 day mission of the Space Life Sciences-1, forced vital capacity and forced expiratory volume in 1s were significantly reduced on flight day 2, but were greater than pre-flight values at day 9. In comparison with standing pre-flight values, tidal volume was decreased by 15% (110 ml) in microgravity and this reduction remained during the entire space flight. Functional residual capacity and expiratory reserve volume decreased significantly in-flight by 520 ml and 370 ml, respectively, when compared with pre-flight standing values. Residual volume was less during flight by 350 ml, when compared with standing control values. This 20% reduction in the residual volume was unexpected as it is normally fairly resistant to change. Lung volumes are believed to be affected by the changes in the intra-thoracic blood volume that occurs throughout the mission, and by alterations in respiratory mechanics and the cranial displacement of the diaphragm and abdominal content that happens in the absence of gravity (2). The gravitational gradient affects the distribution of ventilation and perfusion in the upright human lung. This uneven distribution of ventilation and blood flow within the lungs leads to variations in ventilation-perfusion ratios. Cardiogenic oscillations of CO2 decreased to approximately 60% in amplitude in microgravity (3) and there was also a significant reduction in cardiogenic oscillations of nitrogen (to 44%) and argon (to 24%) in comparison to the pre-flight standing values (4). Possible causes of the residual inhomogeneity of ventilation include regional differences in lung compliance, airway resistance and the motion of the chest wall and diaphragm. Microgravity was expected to abolish completely apicobasal differences in perfusion and its persistence is possibly related to other mechanisms not affected by gravity, such as central-peripheral differences in blood flow and interregional differences in conductance. The diffusion capacity of the lung increased by 62% in a parabolic flight study and by 28% in sustained microgravity when values were compared with pre-flight standing values (5)(6). The standing-to-supine transition pre and post-flight caused a significant elevation in blood volume in pulmonary capillaries. Diffusing capacity of the membrane was unchanged pre-flight in the standing-to-supine transition and significantly elevated in-flight in comparison to standing (27%) and supine (21%). In microgravity, the capillary filling is uniform, which is associated with a large increase in the surface area of the blood-gas barrier. Consequently, the membrane diffusing capacity is substantially raised. This suggests an absence of sub-clinical interstitial pulmonary oedema in microgravity, as had been previously speculated (5)(6). The overall effect of acute and sustained exposure to microgravity, although affecting the respiratory system, does not cause any deleterious effects to the gas exchange in the lungs. However, there is no current suitable method of accessing arterial blood in space. Consequently, at present, values for blood gas tensions are usually derived from measurements of respiratory gas partial pressures. To this end, the earlobe arterialized blood technique for collecting blood gas tensions has been considered for use in space (7). Access to arterial blood analysis will allow better physiological evaluations and the management of clinical emergencies in a space mission, resulting in increased safety for the crew members involved.