Human Pathophysiological Adaptations to the Space Environment

Abstract

Space is an extreme environment for the human body, where during long-term missions microgravity and high radiation levels represent major threats to crew health. Intriguingly, space flight (SF) imposes on the body of highly selected, well-trained, and healthy individuals (astronauts and cosmonauts) pathophysiological adaptive changes akin to an accelerated aging process and to some diseases. Such effects, becoming manifest over a time span of weeks (i.e., cardiovascular deconditioning) to months (i.e., loss of bone density and muscle atrophy) of exposure to weightlessness, can be reduced through proper countermeasures during SF and in due time are mostly reversible after landing. Based on these considerations, it is increasingly accepted that SF might provide a mechanistic insight into certain pathophysiological processes, a concept of interest to pre-nosological medicine. In this article, we will review the main stress factors encountered in space and their impact on the human body and will also discuss the possible lessons learned with space exploration in reference to human health on Earth. In fact, this is a productive, cross-fertilized, endeavor in which studies performed on Earth yield countermeasures for protection of space crew health, and space research is translated into health measures for Earth-bound population.

Drug discovery and development: the final frontier

“…“Astronauts tend to lose 1% of their BMD a month despite being ultra‐fit, working out and taking antiresorptive drugs such as bisphosphonates,” says Dr Braddock, who also leads respiratory drug development projects for a major pharmaceutical company. Broadly, the rate of BMD loss during a six‐month stay on the ISS is similar to that occurring between the 5th and 6th decade of life on Earth …”

“…Gravity has been a fact of life on Earth ever since the first organisms emerged, probably near undersea hydrothermal vents, 3.77 billion to 4.29 billion years ago . Indeed, every cell seems able to sense changes in gravity …”

“…The first tetrapods to tentatively explore land 370 million years ago evolved numerous adaptations to support body weight, regulate fluid and move . One early tetrapod ( Ichthyostega ), for example, evolved bony projections that interlocked its vertebrae to help support its weight as it walked through swamps…”

“…But the ISS orbits at about 28,000km per hour, so its acceleration matches the Earth's curvature. This counterbalances gravity, thereby creating ‘weightlessness’ …”

“…Numerous studies show that microgravity produces profound physiological changes in plants, microbes and mammals. Astronauts, for instance, show a rapid decline in bone mineral density (BMD), especially at sites supporting body weight, such as lumbar spine, femoral neck and trochanter, pelvis, calcaneus and leg . “Astronauts tend to lose 1% of their BMD a month despite being ultra‐fit, working out and taking antiresorptive drugs such as bisphosphonates,” says Dr Braddock, who also leads respiratory drug development projects for a major pharmaceutical company.…”