Prior to sexual intercourse, the blood pressure is increased in the penis. This causes it to become stiff and erect and it can then be inserted into a woman's vagina. Stimulation of the end of the penis causes muscles along the sperm ducts to contract in a wave-like motion. These contractions squeeze sperm past the seminal vesicle and prostate gland where fluid is added to them. The muscular contractions force the semen out of the penis during ejaculation. Million of sperm leave the urethra in a single ejaculation.

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