Final answer:
Urination occurs involuntarily when the bladder reaches a volume of 300 to 400 mL due to the micturition reflex. This reflex involves both voluntary and involuntary pathways with sensory nerves indicating when the bladder should be emptied. The bladder can typically hold up to 500 to 600 mL of urine.
Step-by-step explanation:
When the bladder is filled between 300 to 400 mL, urination occurs without conscious control in a healthy, average person. The micturition reflex controls this process and involves voluntary and involuntary actions.
While a person starts feeling the urge to urinate at about 150 mL, it can be voluntarily overridden. As the bladder fills beyond this point, the urge intensifies until voluntary control is no longer possible, leading to involuntary urination when the volume of urine in the bladder is within the 300 to 400 mL range.
The bladder has stretchy walls that can accommodate varying volumes of urine and has a capacity of about 500 to 600 mL. Sensory nerves and stretch receptors in the bladder signal when it is time to empty it, which involves the coordination of both the internal and external urethral sphincters. High bladder pressure may also trigger the urge to urinate.