Depending upon the length of time the astronaut spends in space, their weight may be less when they return to the Earth. This is due primarily to the lack of gravitational pull on the astronauts body while they are in space. The bones in the skeletal system become shorter. The muscles in the skeletal muscular system atrophy, or waste away, due to the lack of any resistance against them. Even the heart shrinks in size because it has less resistance against it in pumping the blood through the body. Russian cosmonauts who spend months in space aboard the International Space Station (ISS) are often carried off on stretchers when they return to Earth, because of the decreased physical abilities of the skeletal muscular system. So far, all these conditions have reversed themselves when the astronauts return to the Earth and once again experience Earths gravity. So some weight loss is inevitable, largely due to the lack of gravitational attraction in space.