In space, where gravity is very weak, posture can be maintained without standing on your legs, and there is no need to use your legs to move about. Muscles weaken and bone mass decreases. Astronauts may face decreases in blood volume, orthostatic tolerance, and aerobic capacity while also experiencing increased arrhythmias. Although the cardiovascular system functions well in space, the body does not require as much work from the heart (still a muscle, after all) in microgravity.