The simple act of urination can, without gravity, become a medical emergency requiring catheterization and embarrassing radio consults with flight surgeons. “The urge to go is different in space,” says Weinstein. There is no early warning system as there is on Earth. Gravity causes liquid waste to accumulate on the floor of the bladder. As the bladder fills, stretch receptors are stimulated, alerting the bladder’s owner to the growing volume and delivering an incrementally more insistent signal to go. In zero gravity, the urine doesn’t collect at the bottom of the bladder. Surface tension
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