Why doesn't semen enter the urinary bladder during ejaculation?

• A. The prostate gland secretes a barrier that blocks the bladder.
• B. The detrusor muscle contracts to seal off the bladder.
• C. The external urethral sphincter contracts to block the bladder opening.
• D. The internal urethral sphincter closes to prevent backflow into the bladder.

Answer: (D)

Verified step by step guidance

1. Understand the anatomy of the male reproductive system, specifically the role of the internal urethral sphincter, which is located at the junction of the bladder and the urethra.
2. Learn about the physiological process of ejaculation, which involves the coordination of muscles and sphincters to ensure the proper flow of semen through the urethra.
3. Recognize that during ejaculation, the internal urethral sphincter contracts to prevent retrograde flow of semen into the urinary bladder. This is a protective mechanism to ensure semen exits the body properly.
4. Differentiate the internal urethral sphincter from other structures like the external urethral sphincter, which controls voluntary urination, and the detrusor muscle, which is involved in bladder contraction during urination.
5. Conclude that the internal urethral sphincter plays a critical role in preventing the mixing of semen and urine by closing off the bladder during ejaculation, ensuring the reproductive and urinary systems function independently.