Final answer:
The internal urethral sphincter relaxes as the bladder fills, allowing urine into the urethra. The external urethral sphincter is under voluntary control and allows us to consciously control urination.
Step-by-step explanation:
The internal urethral sphincter is a smooth muscle located at the junction between the bladder and the urethra. It relaxes as the bladder fills to allow urine to flow into the urethra. The external urethral sphincter, on the other hand, is a striated muscle that is controlled by the somatic nervous system and allows voluntary control over urination.
During urination, the detrusor muscle in the bladder wall contracts, while the internal urethral sphincter relaxes and the external urethral sphincter contracts. This coordinated action expels urine from the bladder and into the urethra.
It's important to note that the internal urethral sphincter is under involuntary control, while the external urethral sphincter is under voluntary control. This allows us to consciously control when to urinate.