Convertino V A
Physiology Research Branch, Brooks Air Force Base, TX 78235, USA.
Med Sci Sports Exerc. 1996 Oct;28(10 Suppl):S45-52. doi: 10.1097/00005768-199610000-00033.
Plasma volume is reduced by 10-20% within 24-48 h of exposure to simulated or actual microgravity. The clinical importance of microgravity induced hypovolemia is manifested by its relationship with orthostatic intolerance and reduced maximal oxygen uptake (VO2max) after return to one gravity (1G). Since there is no evidence to suggest that plasma volume reduction during microgravity is associated with thirst or renal dysfunctions, a diuresis induced by an immediate blood volume shift to the central circulation appears responsible for microgravity-induced hypovolemia. Since most astronauts choose to restrict their fluid intake before a space mission, absence of increased urine output during actual space flight may be explained by low central venous pressure (CVP) which accompanies dehydration. Compelling evidence suggests that prolonged reduction in CVP during exposure to microgravity reflects a "resetting" to a lower operating point, which acts to limit plasma volume expansion during attempts to increase fluid intake. In ground based and space flight experiments, successful restoration and maintenance of plasma volume prior to returning to an upright posture may depend upon development of treatments that can return CVP to its baseline IG operating point. Fluid-loading and lower body negative pressure (LBNP) have not proved completely effective in restoring plasma volume, suggesting that they may not provide the stimulus to elevate the CVP operating point. On the other hand, exercise, which can chronically increase CVP, has been effective in expanding plasma volume when combined with adequate dietary intake of fluid and electrolytes. The success of designing experiments to understand the physiological mechanisms of and development of effective counter measures for the control of plasma volume in microgravity and during return to IG will depend upon testing that can be conducted under standardized controlled baseline conditions during both ground-based and space flight investigations.
在暴露于模拟或实际微重力环境的24至48小时内，血浆量减少10 - 20%。微重力诱导的血容量减少的临床重要性体现在其与直立不耐受以及返回正常重力（1G）后最大摄氧量（VO2max）降低的关系上。由于没有证据表明微重力期间血浆量减少与口渴或肾功能障碍有关，因此由即时血容量向中心循环转移引起的利尿似乎是微重力诱导的血容量减少的原因。由于大多数宇航员在太空任务前选择限制液体摄入，实际太空飞行期间尿量没有增加可能是由于脱水伴随的中心静脉压（CVP）降低所致。有力证据表明，暴露于微重力期间CVP的长期降低反映了对较低工作点的“重置”，这在试图增加液体摄入时会限制血浆量的扩张。在地面和太空飞行实验中，在恢复直立姿势之前成功恢复和维持血浆量可能取决于能否开发出能使CVP恢复到其基线1G工作点的治疗方法。液体负荷和下体负压（LBNP）在恢复血浆量方面尚未被证明完全有效，这表明它们可能无法提供提高CVP工作点的刺激。另一方面，运动可以长期增加CVP，当与充足的饮食中液体和电解质摄入相结合时，在扩大血浆量方面已被证明是有效的。设计实验以了解微重力及返回1G期间控制血浆量的生理机制并开发有效对策的成功与否，将取决于在地面和太空飞行研究中能够在标准化控制的基线条件下进行的测试。
