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Cardiovascular: Preparation for and participation in spaceflight activities are associated with changes in the cardiovascular system such as decreased carotid artery distensibility and decreased ventricular mass which may lead to an increased risk of cardiovascular disease. Additionally, astronauts who travel into space multiple times or for longer durations may be at an increased risk across their lifespan.

Bone: During prolonged spaceflight, astronauts are exposed to both microgravity and space radiation, and are at risk for increased skeletal fragility due to bone loss. Evidence from rodent experiments demonstrates that both microgravity and ionizing radiation can cause bone loss due to increased bone-resorbing osteoclasts and decreased bone-forming osteoblasts, although the underlying molecular mechanisms for these changes are not fully understood.

It is now recognized that an unloading of the skeleton, either due to strict bed rest or in zero gravity, leads on average to a 1%-2% reduction in bone mineral density at selected skeletal sites each month. The mechanism by which unloading of the skeleton results in rapid mobilization of calcium stores from the skeleton is not fully understood, but it is thought to be related to down regulation in PTH and 1,25-dihydroxyvitamin D3 production. 

Muscle: It’s not a muscular disease, but a disorder caused by the absence of gravity in the International Space Station which makes it nearly impossible to maintain muscle strength. Muscular atrophy involves the weakening and degradation of muscles due to a lack of physical activity.

It’s not to say that astronauts aren’t active (they are in great physical condition) but in space, certain muscles we use on earth require less contracting as they work in a weightless environment. Without the force of gravity causing the muscles to contract, certain muscles decrease in strength, mobility and size. Studies have shown that astronauts experience up to a 20% loss of muscle mass on spaceflights lasting five to 11 days. 


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