Weightlessness, or zero-gravity, has negative effects on the human body and its tissues. One of its main effect is bone resorption, which is the removal of osseous tissue by osteoclasts (bone cells) due to little muscle contractions since our body and the objects around us are weightless when we are out of the reach of the pull of gravity, as we lift things effortlessly in space. As bone resorption makes our bones thinner and brittle, there is the risk that we get a broken leg or arm with little effort once we have come back from outer space.
There is a second negative effect of zero-gravity; muscle atrophy. Lifting and moving our arms, legs, and head effortlessly makes us lose muscle mass. On the surface of the earth, it is the other way around; we contract our muscles with effort when we lift objects and weights, estimulating the osteoblasts to build up bone tissue and our bones and muscles get thicker and stronger.
Being out there in space also affects our heart muscle as its ventricular walls become thiner and, along with it, there is a drop in the number of red and white blood cells. Being too long out of the pull of gravity also damages the blood vessel walls, specially the tiny ones in our brain cortex and lung alveoli. There is also loss of eyesight and sleep disturbance.