During which phase does the bladder contract and the urethral sphincter relax?

The voiding phase is characterized by the contraction of the bladder and the relaxation of the urethral sphincter, which are critical processes for the expulsion of urine. During this phase, neural signals stimulate the detrusor muscle of the bladder to contract, generating pressure within the bladder that forces urine into the urethra. Simultaneously, the external urethral sphincter, which is normally under voluntary control, relaxes, allowing urine to pass through the urethra and exit the body.

This physiological interaction ensures that the bladder can effectively empty its contents while controlling the release through the sphincter mechanism. The voiding phase is directly linked to the process of urination, marking a distinct departure from the storage or filling phases, where the bladder expands and retains urine, and the sphincter remains tightly closed.