The bladder reports stretch, not just fullness
As urine enters the bladder, its muscular wall expands. Sensory nerves in that wall detect the stretch and send signals through the spinal cord to the brain. Those messages build as the bladder fills, so the brain can register a need to urinate before the organ is near its maximum capacity.
The signal is not a simple full-or-empty alarm. How strongly it feels can vary with the pace of filling and with how attentive you are to the sensation. A sudden urge therefore does not necessarily mean the bladder is full.
The brain helps decide when to go
During storage, the bladder muscle stays mostly relaxed while muscles at its outlet help keep urine in. The brain can usually hold off the reflex to urinate until a suitable moment. When you choose to go, the bladder muscle contracts and the outlet relaxes, allowing urine to pass.
That coordination explains why the first urge is a prompt, not a measurement of remaining capacity: your nervous system is managing a growing signal while the bladder continues to fill.
