3D visualization of a male and female engaged in sexual intercourse reconstructed from a Magnetic Resonance Image (MRI). As humans become sexually excited, the sex organs prepare for coitus through changes in the circulatory and nervous systems. The brain receives signals from the genitals. The hearts hastens, flooding the arteries, the veins constrict. Blood engorges the erectile tissue of the penis and clitoris as well as the testicles, ovaries and labia minora - two thin folds of integument that lie just inside the vestibule of the vagina. Muscles tense. Nipples stiffen. These effects plateau. In a woman, the outer third of the vagina becomes vasoconstricted, moistening, while the inner two thirds expand slightly and the uterus becomes elevated - all in preparation for receiving sperm. Male preejaculate adds lubrication near the cervix. The involuntary nervous system increases breathing and quicken the pulse. Orgasm occurs with a loss of control, a shuddering release. In men, this occurs in two stages. As the intensity builds, reflex centers in the spinal cord send impulses to the genitals, prompting the smooth muscles of the testes, epididymides and vas deferentia to contract and squeeze sperm into the urethra. It's the filling of the urethra that triggers the muscles encasing the base of the penis to contract and force the semen out. Women's orgasms involve the uterus and the outer vagina, including the clitoris. Once excited, women are capable of multiple orgasms.

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