Struggling with high blood pressure? A lengthy stay in space may be just what the doctor ordered.

Unfortunately, the side effects of prolonged weightlessness also include a pronounced blood shift -- from the bottom half of the body to the top.

According to new study by researchers working with NASA, three to six months in space sees the body's blood concentration shift more dramatically than previously thought from the lower organs and extremities to the body's upper half. This the reality of the absence gravity, scientists say -- the force that, on Earth, is constantly pulling inert blood towards the feet.

Scientists arrived at their conclusions after drawing blood samples and conducting a variety of tests before and after astronauts spent three to six months on the International Space Station.

"The discovery is important because during long duration missions, the blood volume burden to the heart could constitute a health problem during future long duration missions," explained lead study author Dr. Peter Norsk, a scientist at the University of Copenhagen and a researcher with the Universities Space Research Association.

"We know that some astronauts experience vision problems some months into spaceflight and this may in fact be caused by the augmented fluid and blood volume shift to the upper body," Norsk said.

NASA has ramped up its efforts to better understand the health impacts of prolonged periods in microgravity on its astronauts; and eyesight problems are one of the most concerning factors. Researchers have found that nearly a quarter of all astronauts experience vision impairment upon their return from space.

"This is one that we don't yet have a good handle on, and it can be a showstopper," Mark Shelhamer, chief scientist for the NASA Human Research Program, said last spring.

Norsk and his colleagues also found that the blood pressure of astronauts was considerably reduced after a lengthy stint in space. The reduction was on par with the effects blood pressure medicine has on most patients battling high blood pressure.

The new research is detailed in the latest issue of The Journal of Physiology.