Other abnormalities of autonomic nervous system regulation may also be present. Many of the autonomic function experiments flown on the recent Neurolab Life Sciences mission were designed to address these and other aspects of neurohumoral control of circulation.

Postflight orthostatic intolerance is also associated with a major decrease in exercise capacity. Maximal heart rates are unchanged compared with preflight test results.^{54 55} Blood pressure at the start of exercise is either unchanged or slightly elevated because of increased vasoconstriction. The major hemodynamic defect is an inadequate stroke volume, which is decreased by one-third or more from preflight levels and leads to proportionate decreases in cardiac output and skeletal muscle oxygen delivery. Undoubtedly, some component of skeletal muscle atrophy and decreased neuromuscular coordination contribute to the decreased aerobic capacity, but these are probably minor factors compared with decreased blood volume and autonomic nervous system dysregulation. Operational concerns about postflight orthostatic intolerance and aerobic deconditioning have increased since astronauts began using the heavy, bulky, partial-pressure launch and entry suit (LES) now required for all post-Challenger flights.

Recovery to preflight levels of orthostatic tolerance occurs within a day or so following flights of less than 1 month's duration, but longer recovery is associated with longer-duration flights.⁵⁶ Recovery of aerobic capacity is also relatively rapid but takes about a week or so after landing. The time course of cardiovascular recovery consistent with return of autonomic control and intravascular volume is too fast for recovery of skeletal muscle atrophy. The recovery course is also slower than was believed from clinical observations only, pointing out the need for careful, objective measurements of physiological function and capacity for accurate assessment of functional status and capability. Serial, long-term data on recovery following long-duration flights are lacking. Magnetic resonance imaging has documented a 7 to 10 percent decrease in heart muscle mass following spaceflights as short as 10 days.^{57 58 59} The significance of this finding and its recovery time course are unknown.

A variety of cardiovascular countermeasures have been proposed and/or instituted to counteract the changes associated with spaceflight. ^{60 61 62 63 64 65 66 67 68} Most of these countermeasures have evolved over time and are frequently based on clinical intuition rather than prospectively collected data. The Russians use thigh constriction cuffs (braselet) to decrease cephalad fluid shifts at various phases of their long-duration spaceflights (Oleg Yu Atkov, M.D., Cosmonaut-USSR, personal communication). Data on the effectiveness of these cuffs are lacking, but many of the cosmonauts report significant relief from the head congestion and facial edema otherwise associated with microgravity. An aggressive in-flight exercise program seems to be only partially effective in maintaining postflight aerobic capacity, and its effects on orthostatic tolerance are largely unknown. On the Russian space station Mir, cosmonauts and astronauts currently exercise for almost 2 hours daily during flight and use saline loading and anti-g garments to minimize orthostatic intolerance postflight. On the U.S. shuttle, astronauts tend to perform some aerobic exercise, although there is no formal requirement for exercise during flight, and both exercise and fluid-loading regimens differ according to crew preference, so that actual practice is highly variable.

Analysis of the physiological responses to large amounts of intravenous normal saline solution administered to bed-rest subjects or to astronauts in-flight showed that the adapted blood volumes were maintained at their reduced bed-rest or 0-g levels and that the saline was cleared from the circulation of these reduced-blood-volume subjects as rapidly as in 1 g.^{69 70} Anecdotal reports suggest that much of the fluid load taken by shuttle astronauts just prior to reentry is also rapidly cleared by urination. The

Front Matter (R1-R20)

Executive Summary (1-20)

1 Introduction (21-28)

2 Cell Biology (29-36)

3 Developmental Biology (37-48)

4 Plants, Gravity, and Space (49-62)

5 Sensorimotor Integration (63-79)

6 Bone Physiology (80-96)

7 Skeletal Muscle (97-117)

8 Cardiovascular and Pulmonary Systems (118-131)

9 Endocrinology (132-155)

10 Immunology (156-170)

11 Radiation Hazards (171-193)

12 Behavioral Issues (194-230)

13 Setting Priorities in Research (231-236)

14 Programmatic and Policy Issues (237-250)

Appendix A: Acronyms and Abbreviations (251-252)

Appendix B: Glossary (253-265)

Appendix C: Workshops (266-271)

Appendix D: Committee Biographies (272-276)