An increase in food intake, combined with vigorous inflight exercise using a variety of equipment, can at least partially counteract muscle deterioration. The redistribution of body fluids during weightlessness causes the body to eliminate some fluids in the urine. After the return to Earth, body fluids return to preflight norms rapidly and blood counts return to normal within a few weeks. The shift in body fluids and the decrease in blood volume, along with the absence of gravity, also reduce the size and pumping capacity of the heart. Exercise and adequate fluid intake are, again, the usual countermeasures used to avoid serious cardiac and circulatory problems.
Most cardiovascular responses return to normal within a few weeks.
Loss of calcium from the weight-bearing bones of the body is apparently continuous throughout a flight. The loss is significant, ranging in various individuals from 0.5 percent to 1.5 percent total body calcium per month. Postflight recovery of lost calcium is a lengthy process, taking at least as long as the flight itself. Such changes indicate possible changes in the body's immune system over long periods of time. Finally, some indications exist that drugs commonly used to treat illnesses on Earth may not be absorbed properly by the body in weightless conditions.