How is urine kept from coming out of the urethra when a male ejaculates?
When a man becomes sexually aroused his body
makes several adjustments in preparation for sexual intercourse.
In his prostate gland, there is a fork in the urethra. One side leads
up to the bladder where urine is stored. The other side leads up
to the ejaculatory duct. For most of a man's life, the passage to
the bladder is open and the passage to the ejaculatory duct is squeezed
close. This prevents urine from back flowing into the ejaculatory
duct where the acidic urine would kill his sperm. When a man is sexually
aroused, muscles in the prostate gland change the openings. The
tube to the bladder is squeezed closed and the tube to the ejaculatory
duct opens up. If you have ever seen a railroad track switch, the
change operates in a similar fashion. The male body is designed so
that only one passage is open at a time. Hence, when a man is strongly
aroused, he is unable to urinate.