Human Response to Space Flight

The past five decades have amply demonstrated that humans can tolerate space flight well for long periods in orbiting spacecraft. Historically, the direct causes of mortality have been accidents occurring during dynamic phases of flight. The vast majority of flight time has been spent in Earth orbit, but both in orbit and on the lunar surface, humans have demonstrated the ability to maintain adequate health and to work productively. The dominant condition associated with Earth orbit affecting human physiology and health is weightlessness, which induces predictable changes in crewmembers during adaptation. Acutely, these changes can induce adverse symptoms such as space motion sickness from neurovestibular adaptation and facial congestion associated with a rostral fluid shift. Typically these symptoms do not limit crew activity and resolve within a few days. Significant but clinically asymptomatic early changes include regulation to a lower plasma volume with a concomitant decrease in red blood cell mass, changes in cardiac and respiratory dynamics, and changes in anthropometry. Food intake is volitionally reduced and weight loss is common. Changes in skeletal muscle morphology are seen, and muscle mass and strength in postural regions are reduced after several days. Aerobic fitness is reduced but does not limit inflight performance.