The stretch receptor nerves lining the bladder prompt urges to urinate when they sense the bladder is about one-third full. However, urine does not collect at the bottom of the bladder in low gravity environments. In space, the urge is only triggered once the bladder is almost totally filled. This could cause the urethra to be forced shut, making urination very difficult. Astronauts must take precaution against this. Source: Mary Roach, "Packing for Mars: the Curious Science of Life in the Void"