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Space affects the body in many ways. A partial list of the consequences of long stays in microgravity (where the pull of Earth's gravity is virtually unnoticeable to humans) includes bone loss at a rate of 1 to 1.5 percent a month, producing changes similar to osteoporosis; an increased risk of kidney stones and bone fractures, both associated with bone demineralization; and loss of muscle mass, strength and endurance, especially in the lower extremities. Other changes are diminished cardiac function and the possible occurrence of heart rhythm disturbances, redistribution of body fluids away from the extremities and toward the head, and alterations in the neurovestibular system that often lead to disorientation and decreased neuromuscular coordination on return from prolonged missions. Disruptions of circadian rhythms because the 24-hour day-night cycle is absent result in sleep loss and stress, and the body experiences reduced blood volume, immunodeficiency and transient postflight decreases in levels of red blood cells, despite adequate nutritional intake.

Space also presents health risks in the form of radiation, normally blocked by Earth's atmosphere. The space environment contains galactic cosmic rays, heavy ions such as iron, trapped electrons and protons, and neutrons. Such radiation can induce cataracts and cancer and adversely affect physiological processes.