A shift in the distribution of human body fluids due to the effects of microgravity. In the early stages of a space flight, an astronaut experiences a puffy face, stuffy nose, and headaches due to the shift of fluids from the lower body and the dilation of cranial veins. Body mechanisms are then activated that lead to a significant loss of water and body mass. During longer periods in space, however, the cardiovascular system adapts to the microgravity environment. When an astronaut returns to Earth, the fluid shifts back to the lower body, and this can cause orthostatic hypotension and loss of consciousness.