## 1 Introduction
### Effects on the Pulmonary System  
During short-duration spaceflight, a host of changes to normal, physiologic pulmonary function have been observed [73; 227]. Studies during parabolic flight have shown that the diaphragm and abdomen are displaced cranially due to microgravity, which is accompanied by an increase in the diameter of the lower rib cage with outward movement. Due to the observed changes to the shape of the chest wall, diaphragm, and abdomen, alterations to the pressure-volume curve resulted in a net reduction in lung volumes [228]. In five subjects who underwent 25 s of microgravity exposure during parabolic flight, functional residual capacity (FRC) and vital capacity (VC) were found to be reduced [229]. During the Spacelab Life Sciences-1 mission, microgravity exposure resulted in 10%, 15%, 10-20%, and 18% reductions in VC, FRC, expiratory reserve volume (ERV), and residual volume (RV), respectively, compared to values seen in Earth's gravity [227]. The observed physiologicchange in FRC is primarily due to the cranial shift of the diaphragm and abdominal contents described previously, and secondarily to an increase in intra-thoracic blood volume and more uniform alveolar expansion [227].  
One surrogate measure for the inhomogeneity of pulmonary perfusion can be assessed through changes in cardiogenic oscillations of CO\({}_{2}\) (oscillations in exhaled gas composition due to differential flows from different lung regions with differing gas composition). Following exposure to microgravity, the size of cardiogenic oscillations were significantly reduced to 60% in comparison to the preflight standing values [230; 231]. Possible causes of the observed inhomogeneity of ventilation include regional differences in lung compliance, airway resistance, and variations in motion of the chest wall and diaphragm. Access to arterial blood gas analysis would allow for enhanced physiologic evaluations, as well as improved management of clinical emergencies (e.g., pulmonary embolism) occurring during space travel. However, there is currently no suitable method for assessing arterial blood in space. The earlobe arterialized blood technique for collecting blood gas has been proposed, but evidence is limited [232]. Further research is required in this area to establish an effective means for sampling arterial blood during spaceflight.  
In comparison to the changes seen during short-duration spaceflight, studies conducted during long-duration spaceflight showed that the heterogeneity of ventilation/perfusion (V/Q) was largely unchanged, with preserved gas exchange, VC, and respiratory muscle strength [73; 233; 234]. This resulted in overall normal lung function. This is supported by long-duration studies (up to 6 months) in microgravity which demonstrated that the function of the normal human lungs is largely unchanged following the removal of gravity [233; 234]. It is worth noting that there were some small changes which were observed (e.g., an increase in ERV in the standing posture) following long-duration spaceflight, which can perhaps be attributed to a reduction in circulating blood volume [233; 234]. However, while microgravity can causes temporary changes in lung function, these changes were reversible upon return to Earth's gravity (even after 6 months of exposure to microgravity). Based on the currently available data, the overall effect of acute and sustained exposure to microgravity does not appear to cause any deleterious effects to gas exchange in the lungs. However, the biggest challenge for long-duration spaceflight is perhaps extraterrestrial dust exposure. Further research is required to identify the long term consequences of extraterrestrial dust exposure and develop potential countermeasures (e.g., specialized face masks) [73].