An erection is dependent upon finely orchestrated actions within penile tissues, nerves, and blood vessels. Additionally, the lifting action and maintenance of a firm erection requires good blood flow—which is regulated by the central nervous system—and the ability to trap the blood in penile spongy tissue.
Erections that occur during normal sexual activity begin in the conscious brain, with a nervous system response to either real or imagined erotic stimulation. This leads to a change in the penis from flaccid (soft) to tumescent (swollen) to erect (rigid).
The shaft of the penis holds two individual chambers called the corpora cavernosa. An expandable, spongy tissue fills the chambers, extending from the base to the tip of the organ. This tissue contains blood vessels and smooth muscles. The urethra, the channel for urine and ejaculate, runs along the underside of the corpora cavernosa. A membrane, called the tunica albuginea, surrounds the corpora. In the normal, flaccid state, the smooth muscle constricts the blood vessels and keeps blood out of the penis.
An erection is initiated when the brain senses something arousing. Impulses are sent from the brain to the lower part of the spinal cord, through the pelvic nerves, 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. Blood flow has to increase to about six times its normal rate to fill the penis and cause it to become longer, wider, and harder. Like a sponge, the corporal tissues then quickly expand with blood, engorging and enlarging the penis.
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 within the corporal bodies by creating a fixed resistance, allowing an increased pressure to occur.
Finally, engorged 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 outflow prevented, the erection will be sustained.