Table 1.

Timeline of physiologic acclimation and acclimatization experienced by astronauts from launch to after return to earth

		Duration of flight				Postflight period
Physiologic effects	Launch	24 h	48 h 2 wk	> 1 mo	Landing	24–48 h	1–2 mo	> 1 yr
Fluid redistribution	Redistribution of fluid to the torso and head 10% decreased fluid volume in the legs	17% reduction in plasma volume	Gradual decrease in erythropoietin secretion, leading to a 10% decrease in total blood volume		Orthostatic hypotension from pooling of fluids in the legs	Return of normal fluid distribution
Neurovestibular effects	Space motion sic kness				Space motion sickness
Muscle changes		Gradual decrease in muscle mass by 20%		Gradual decrease in muscle mass by 30%	Muscle soreness and tightness		Full recovery of muscle mass and strength
		Gradual decrease in muscle strength (up to 50% loss observed)
Bone demineralization		60%–70% increase in calcium loss (urinary, fecal). Reduced parathyroid hormone and vitamin D production.						Complete or almost complete restoration of bone density
		Gradual loss of bone density (1%–2% per month)
Psychosocial effects	Fatigue, sleep debt, isolation, emotional effects, stress to the astronaut’s family, multicultural crew environment
Immune dysregulation		Possible reactivation of latent herpes viruses and impairment of cell-mediated immunity			Numerous cellular and other changes leading to impaired immunity		Gradual improvement in immunity (days to weeks)