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decreases CO2 clearance from the body and can have profound effects on astronauts' health and function.44 Further studies should be performed to determine acceptable spacecraft CO2 levels.
Pulmonary gas mixing and alveolar ventilation were assessed by examining the concentrations of exhaled helium (He) and sulfur hexafluride (SF6).45 These gases, like all respiratory gases, have similar convective mixing properties, but their molecular weights are 4 and 146, respectively, and so the diffusivity of He is approximately 6 times that of SF6. Convective mixing of inspired gases occurs mainly in the centrally located alveoli, while peripheral acinar ventilation is much more dependent on diffusion. When measured in both standing and supine subjects on Earth, mixing inhomogeneities were much greater for SF 6, whereas in sustained microgravity, the differences disappeared. During breathholding in microgravity, SF6 actually exhibited better mixing properties than did He. These results were quite surprising and contradicted a widely accepted component of pulmonary physiology. The investigators were not able to explain their findings fully but postulated that changes in acinar architecture produced by the absence of gravity may have altered ventilation, especially in the peripheral regions. It is also possible that alterations in gas mixing caused by the normal cyclical compression of the lungs by the heart may also have played a role. Similar changes were not seen when measurements were made during short periods of microgravity (20 to 25 seconds) produced by parabolic flights.46 All of the observed changes in pulmonary function returned to the preflight baseline almost immediately after reentry to 1 g, whereas cardiovascular readaptation to 1 g appears to be slower and is associated with physiologically and operationally significant impairment. Aerosol deposition has been studied only during parabolic flight, but measurements taken then show unexpectedly high deposition of small particles compared to predictions. This suggests that the absence of sedimentation allows particles to travel more deeply into the alveolar region of the lung where they deposit. 47 Although the absence of sedimentation still reduces overall deposition, the potential health effects of deeper particle penetration are unknown.
Postflight Cardiovascular Physiology
Orthostatic intolerance, an impaired ability to maintain adequate blood pressure while in an upright position, has been found in almost all astronauts returning from spaceflights of even a few hours' duration. Overall, about two-thirds of the astronauts tested early postflight had hemodynamically significant orthostatic intolerance.48 Virtually no testing has been performed on shuttle pilots and flight engineers during reentry, but levels of postflight impairment suggest that orthostatic intolerance remains a major, unresolved, clinical and operational problem. The extent of orthostatic intolerance postflight is variable and depends on the duration of flight, interindividual differences in cardiovascular function among the astronauts, and the time and method of postflight testing. Careful hemodynamic measurements of astronauts immediately postflight show that heart rate and stroke volume are relatively well maintained initially, but a failure to increase total peripheral resistance adequately leads to a fall in blood pressure, insufficient brain blood flow, and an inability to stand quietly for more than 5 to 10 minutes postflight.
Decreased red blood cell mass and plasma volume (see the section "Hematology" in Chapter 9) is a major factor but does not fully explain the extent of cardiovascular deconditioning seen postflight. Excessive pooling of blood in the lower extremities, thought likely because of decreased mass and tone in postural muscles, does not seem to be present.495051 Carotid baroreflex function is decreased in-flight and remains so for some days after return to Earth, but the extent to which it accounts for the observed orthostatic intolerance is not clear. Inadequate vasoconstriction may be due to baroreflex dysfunction, but in-flight experiments thus far have addressed only the heart-rate component of the baroreflex.5253
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