sm-220-astronaut-spacewalk-00710756

Spacewalks are gruelling for astronauts because they must work non-stop and have no access to food while they are outside the space station. (Ron Garan/NASA/Associated Press)

Astronauts get to float weightless and enjoy a unique view of the world along with other extraordinary experiences. But working in space also takes its toll on the human body.

Canadian astronaut Chris Hadfield has described the physical challenges faced by spacewalkers, starting with putting on the specialized spacesuit required to provide oxygen and pressure to the body and protect it from extreme temperatures, and high-speed objects flying through space.

During training, you need "great big burly guys to help squeeze your body into the suit," Hadfield said during a 2011 presentation to the Royal Astronomical Society of Canada. "The suit is really uncomfortable and very hard on the body. You normally come out of it bloody."

The spacewalk itself is grueling because astronauts work non-stop and have no access to food while they are outside the space station.

Weightlessness itself also takes a toll.

"The first time you pee in space, your urine is full of your skeleton," Hadfield said, adding that the human body devotes less energy to building a tough structure to fight gravity if it doesn't seem necessary.

However, research has found that if astronauts engage in certain kinds of resistive exercise while in space, they come home with "virtually zero bone loss," Hadfield added, so astronauts now each work out two hours a day.

Scientists are now discovering other negative health effects of spending long periods in space. Recent studies show astronauts' vision deteriorates after a long stay in space. Hadfield said age may be a factor, as many astronauts are in their 40s and 50s. Weightlessness, which tends to shift more fluid from the lower to the upper body and the astronaut diet, which tends to be high in salt, may also affect the eyes by changing the distribution of fluid around the eyeball and the optic nerve.

But Hadfield said scientists aren't yet sure what causes the vision problems. "We're working on it," he said.