Which organ has sphincters that relax to allow urine to drain into the urethra?

The urinary bladder is the organ responsible for storing urine produced by the kidneys until it is ready to be excreted. It has muscular walls and sphincters that control the flow of urine. The internal and external urethral sphincters play a critical role in this process. When the bladder fills to a certain volume, stretch receptors signal the nervous system, leading to the relaxation of these sphincters. This relaxation allows urine to flow from the bladder into the urethra, marking the first step in the process of urination.

In contrast, the kidneys are responsible for filtering blood to form urine, but they do not have sphincters involved in urine drainage. The urethra itself is the duct through which urine exits the body, but it does not contain sphincters that control urine flow from the bladder. The prostate gland, which is involved in the reproductive system, surrounds part of the urethra but does not function to regulate the release of urine from the bladder. Therefore, the role of the urinary bladder and its sphincters is crucial for the controlled expulsion of urine.