Ejaculation is the ejection of semen out of the urethra (passageway inside the penis) when a man has an orgasm. Under normal circumstances, ejaculation propels semen forward through a man's urethra and out the tip of his penis. This is because a tiny sphincter (circular muscle) at the entrance to the bladder shuts the opening to the bladder and prevents semen from entering. Retrograde ejaculation is when the semen travels backwards into the bladder.
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