Sexual intercourse is the process by which sperm is delivered to the female reproductive tract. If a female is fertile, a sperm cell can fertilize the female gamete and produce a diploid zygote. During sexual intercourse, a male must be sexually aroused to initiate a penile erection. An erection is due to an increased firing of parasympathetic nerves, which prompts release of nitric oxide. Nitric oxide causes blood vessels serving the spongy tissue of the penis to widen, allowing blood to fill the tissue and the penis to become erect. An erect penis allows a male to penetrate a female's vagina during sexual intercourse. This can lead to ejaculation of seminal fluid containing sperm to the outside of the male's body and into the female's reproductive tract. There are four general stages of sexual intercourse:
- Excitement: this is the beginning phase of arousal. Signs of excitement range from an increase in heart rate to accelerated breathing and swelling of the testes or tightening of the scrotum. Erection of the penis occurs as the spongy tissue fills with blood.
- Plateau: this stage continues until ejaculation. During the plateau phase the penis and testes continue to increase in size, the testes continue to ascend, and pre-ejaculation secretions neutralize the pH of the urethra.
- Orgasm and ejaculation: recognized as the climax stage, this is the shortest phase of sexual intercourse. It lasts only a few seconds and is characterized as a period where muscles involuntarily contract to generate spasms, the heart rate reaches a peak, and continual contractions at the base of the penis occur. Semen is ejaculated during an orgasm. During ejaculation, sperm move from the tail of the epididymis and through the ductus deferens and combine with seminal fluids in the urethra. Ejaculation is under sympathetic neural control. Orgasm is the pleasurable sensations that occur during ejaculation.
- Resolution: This phase generally results in a feeling of relaxation as the body returns to an unexcited stage. Resolution starts after orgasm ends and only if no additional stimulation is provided. Stimulation by the sympathetic nervous system causes blood to flow out of the penis, returning the penis to its flaccid state.