An erection is dependent upon the finely orchestrated actions of muscles, nerves, and blood vessels in the penis. Additionally, it requires good blood flow, which is regulated by the nervous system, to bring about the hydraulic, or lifting, action.
Three to six spontaneous erections occur without erotic stimulation every night during REM (rapid eye movement), or dream, sleep. Erections which take place during normal sexual activity, however, begin in the male’s conscious brain with a nervous system response to either real or imagined erotic stimulation. The change in the penis from flaccid (soft) to tumescent (swollen) to erect (rigid) is caused by an intricate partnership involving the brain, blood vessels, nerves, and hormones.
The shaft of the penis holds two individual chambers called the corpora cavernosa. A spongy tissue constructed of thousands of expandable saclike structures fills the chambers, which extend from the base to the tip of the organ. This tissue also contains blood vessels and smooth muscles. The urethra, the channel for urine and ejaculate, runs on the underside of the corpora cavernosa, while a membrane, known as the tunica albuginea, surrounds the corpora. In the normal, flaccid state, the smooth muscle keeps the blood vessels constricted, keeping blood out and the penis soft.
An erection begins when the brain senses something arousing.
Impulses are then sent from the brain to the lower part of the back, (through the pelvis, and to the penis. Nerve stimulation, most likely induced by nitric oxide, a gaseous molecule, causes the smooth muscles of the penis to relax. This allows increased quantities of blood to flow in through the right and left cavernosal arteries, filling the space within the cavernosa. For the blood to fill the penis and cause it to become longer, wider, and harder, it has to multiply to about six times its normal flow. Like a sponge, the corpora tissues quickly expand with blood, engorging and enlarging the penis.
Then, as the corpora cavernosa continue to swell, they press against the veins that normally allow blood to flow out, effectively preventing it from leaving. The tunica albuginea also helps to trap blood there, with the result that pressure is built. Finally, packed with blood, the corpora become rigid and erect, making the penis firm enough for penetration. As long as the inflow of blood is maintained, and the outflow is prevented, the erection will be sustained.
All of these actions happen automatically; men have no voluntary control over these mechanical forces.
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