How Sex Works
by Craig Freudenrich, Ph.D.
Fertilization
Delivering Sperm Cells
Female Sex Organs
Development of Sex Organs
Other Sexual Organs
Production of Sex Cells
Reproductive Cycles
Intercourse
When a man and a woman engage in sexual activity, each goes through an arousal phase. In both the man and the woman, nerve impulses from the brain cause their heart rates to increase and dilate peripheral blood vessels. They feel warm, and they begin to sweat. The Cowper's glands in the man and the vestibular glands in the woman secrete fluid that lubricates the man's urethra and the woman's labial area and vagina.
The man's brain sends nerve impulses to the blood vessels in his penis and tells the arterioles to dilate and the venules to constrict. The blood flow engorges the spongy tissue of his penis, causing it to become erect. As the couple engages in intercourse, the man inserts his erect penis into the woman's vagina. As intercourse continues, the man reaches a point at which muscle contractions in the epididymis, prostate and seminal vesicles propel semen from the penis into the woman's vagina (ejaculation) at the base of the uterine cervix. Muscle contractions in the woman's body periodically dip her cervix into the semen.
Once the semen is deposited at the base of the uterus, the sperm begin a long journey to fertilization.
source: how stuff works
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