What internal factor primarily governs the psychological *urge* to urinate on Earth?

Bladder stretch receptors triggering pressure buildup

The internal biological sensation or *urge* to urinate is governed primarily by the physical stretching of the bladder walls. Stretch receptors located in the bladder tissue detect increasing volume and pressure, sending signals to the brain indicating that voiding is necessary, a process largely independent of external forces like gravity. While gravity assists in the physical act of emptying on Earth, the psychological signal of urgency arises from this internal stretching. In space, this internal signal remains, but the astronaut must consciously override the gravity assist and rely entirely on mechanical suction for the actual physical release.