What determines when you need to urinate?

The need to urinate is primarily determined by the bladder's capacity and the accumulation of urine produced by the kidneys. As the bladder fills with urine, stretch receptors signal the brain, creating the sensation of urgency. Factors such as fluid intake, hydration levels, and certain medical conditions can influence both the volume of urine produced and the frequency of urination. Additionally, individual bladder sensitivity and habits play a role in determining when one feels the need to urinate.