What are some effects of zero gravity on the human body?

Zero gravity affects the human body in several ways. When people spend time in weightless conditions, their muscles and bones can become weaker because they do not need to support their weight. Fluids in the body move upward, making astronauts' faces look puffy and legs thinner. Bones lose calcium, and blood might not flow as strongly to the legs. There can be a loss of balance and dizziness when returning to Earth. Exercise is important for astronauts to stay healthy. Most effects go away after coming back, but bone loss can last longer and might require medical care.​