Final answer:
The urge to void occurs when the bladder contains 150 mL of urine. This urge, part of the micturition reflex, intensifies as the bladder fills and can typically be overridden initially, but may lead to incontinence as bladder volume increases.
Step-by-step explanation:
The urge to void typically occurs when the bladder contains as little as 150 mL of urine. The micturition reflex, which is the process of urination, is an interplay of both involuntary and voluntary actions controlled by the internal and external urethral sphincters. Initially, the urge to urinate at 150 mL can be consciously overridden by preventing the relaxation of the external urethral sphincter. As the bladder continues to fill, these urges become more persistent and difficult to ignore. Eventually, when the bladder volume approaches approximately 300 to 400 mL, maintaining urinary continence can fail, resulting in incontinence.
In the functioning of the renal system, nephrons filter a large volume of body fluids, ultimately producing urine that is then stored in the bladder. The capacity of the bladder to hold urine varies, often accommodating approximately 500 to 600 mL before reaching a point where urination cannot be delayed. The mechanics and control of micturition change with age, and the ability of the detrusor muscle to contract forcefully decreases, necessitating voluntary contractions of abdominal muscles for complete bladder emptying.