The human heart shrivels away in space, but researchers have found that mini-hearts grown from human stem cells sprout in space significantly faster than in labs on Earth.
Weird things happen to astronauts' hearts in microgravity. Without the sense of up and down, the flow of blood in the body changes. More of the fluid gathers in the head, and there is suddenly less of it not just in the legs but also in the heart itself. Not having to push the body against the resistance of gravity, the heart shrinks, weakens and even changes its shape, becoming more circular.
