What physiological event occurs shortly after reaching orbit causing facial changes?
Answer
Cephalad fluid shift
The cephalad fluid shift is the most significant change observed immediately upon entering orbit. Without the constant downward pull of Earth's gravity, bodily fluids redistribute away from the lower extremities and abdomen, moving toward the chest and head region. This centralization of fluid volume causes observable physical manifestations such as the characteristic 'puffy face' and 'neck bulge' experienced by astronauts, contrasting sharply with the thinner appearance of their legs. While the total body water volume may remain stable or slightly decrease over time, this initial redistribution is interpreted by the body as central fluid excess.
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