Biomedical Research 



the cardiovascular system sense the 

 redistribution of blood volume and activate 

 regulatory mechanisms that counteract this 

 "central blood volume expansion" by reducing 

 the total blood volume. 



For example, the human body can decrease 

 blood volume by increasing urine output, 

 decreasing thirst, or changing the permeability 

 of blood vessels to fluid. These changes may be 

 mediated by humoral agents, such as atrial 

 natriuretic factor, vasopressin, angiotensin, 

 aldosterone, and catecholamines; and these 

 physiologic changes may also be reflected in 

 hematologic indices, such as the hematocrit 

 (percent volume of red blood cells/total blood 

 volume). Redistribution of blood flow, reduced 

 blood volume, and reduced cardiac output on 

 short-duration missions may contribute to the 

 deconditioning observed in astronauts on their 

 return to Earth. Upon reentry into full gravity, 

 the vascular pooling of blood in the lower 

 extremities results in a decrease in blood flow to 

 the cerebral vasculature and may result in 

 syncope (loss of consciousness) if appropriate 

 countermeasures are not taken (4, 5, 6, 8, 

 9, 10, 11). 



In addition, many astronauts experience space 



adaptation syndrome, characterized by nausea, 



emesis, nasal congestion, diaphoresis (sweating), 



and fatigue. Signs or symptoms of space motion 



sickness, which can be incapacitating, have been 



reported in a significant number of astronauts: 



11 out of 33 Apollo astronauts, 5 out of 9 Skylab crew members, and 6 out of 12 



early Shuttle crew members (9). 



Other cardiovascular concerns are related to space flight. Acute acceleration has 

 been associated with cardiac dysrhythmias, particularly bradycardia. A broad range 

 of arrhythmias, including both atrial and ventricular arrhythmias, has occurred 

 among astronauts during space flight. It has been reported that a Soviet 

 cosmonaut was recently rescued after 6 months in space because of cardiac 

 dysrhythmias. Little information exists on the prevalence and pathophysiology of 

 cardiac arrhythmias in space, a concern especially important for long-term 

 missions. 



Other aspects of cardiovascular deconditioning have been reported during space 

 flight. These involve complex alterations in cardiovascular reflexes, endocrine 



Astronaut Owen Garriott, at left, draws blood from felloio crew 

 member Byron Lichtenberg during a biomedical research experi- 

 ment aboard Spacelab 1. 



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