| In reduced gravity, the human heart does not have to work hard against the full force of Earth's gravity to pump blood to all the tissues. Like any other muscle, the heart shrinks in size with decreased demand and during times of exertion, can't get sufficient oxygenated blood to the muscles, nor can it deliver enough blood to the skin to regulate body temperature. Over time, astronauts experience reduced aerobic capacity and impaired heat tolerance that could potentially present serious risks during physically demanding tasks such as space-walks. A space-adapted heart can be especially challenged when gravity is fully restored, rendering many astronauts prone to fainting, at least in part because the heart can't pump enough blood to the brain.
Even if an astronaut stays fit, a cardiac event (like a heart attack or new onset angina), which occurs in space would likely be catastrophic since expert medical care is not accessible. To prevent this from happening, astronauts are carefully and continuously screened for predisposition to cardiovascular disease before and after acceptance into the space program. However, the space environment (which includes not only reduced gravity but also space radiation exposure) may accelerate degenerative diseases such as atherosclerosis. |
