HOUSTON --NASA's new focus will soon be getting people to Mars. But NASA has some major health concerns to figure out, like what happens to the body when it spends a prolonged time in space. One astronaut who spent months at the International Space Station is now the focus of medical tests. If Mars is NASA'S new mission, it will take more than hardware and software to get there. Humans will be along for the long ride, so what will happen to their bodies after years in space? First, the heart changes. "Our hearts don't have to pump blood all the way down, all the way up and so we see changes in how the heart behaves in space. And we're interested in what that means for people when we get back home," Astronaut Cady Coleman said. Coleman just returned from a six-month stay on the space station. "I feel actually just normal except a little more tired, Coleman said. Cady had an MRI scan before and after her space station stay for a NASA/UTHealth study. They're watching her heart because they already know the human heart can shrink by 25 percent. They call it the Grinch Syndrome. "There have been some heart rhythm abnormalities also noted in space flight," said Dr. Michael Bungo, who's working on the study. Fainting and tunnel vision can happen to astronauts like Coleman after six months in space. A mission to Mars could take up to two years. "Real people will go on those journeys, parents that have kids at home who want to come home to them," Coleman said. Zero gravity affects the heart much like bed rest. There's already one study that's helping heart patients on Earth. Thirty minutes of rowing improves the heart as much as 90 minutes of cycling. And that's helpful to astronauts and to people recovering from heart attacks. And Coleman put herself through test after test as a human subject in 25 medical studies, just like this one. "History's not going to remember who Cady Coleman is, but history's certainly going to remember what she's done," Dr. Bungo said. NASA is also studying bone loss, radiation during flight, and the psychological effects of long space flight.