How an Erection Occurs
Basically, an erection illustrates simple hydraulics. Blood fills chambers in the penis, causing it to swell and become firm. But getting to that stage requires extraordinary orchestration of body mechanisms. Blood vessels, nerves, hormones, and, of course, the psyche must work together. A hitch in any one of these elements can diminish the quality of an erection or prevent it from happening altogether.
Directly stimulating the genitals can also prompt an erection, though different nerve pathways are involved. Here, sexual sensation is carried by the pudendal nerve, which runs from the penis to the sacral nerves in the lower spine. The sacral nerves then send messages that cause the arteries in the penis to admit blood. During sexual activity, both of these nerve pathways are involved in producing an erection.
Nerves talk to each other by releasing nitric oxide and other chemical messengers. These messengers boost the production of other important chemicals, including cyclic guanosine monophosphate, prostaglandins, and vasoactive intestinal polypeptide. These chemicals initiate the erection by relaxing the smooth muscle cells lining the tiny arteries that lead to the corpora cavernosa, a pair of flexible cylinders that run the length of the penis (see Figure 3).
Figure 3: Anatomy of the penis
The penis is made up of three cylindrical bodies: the corpus spongiosum (spongy body) — which contains the urethra and includes the glans (head) of the penis — and two corpora cavernosa (erectile bodies), which extend from within the body out to the end of the penis to support an erection. Blood enters the corpora cavernosa through the central arteries.
As the arteries relax, the thousands of tiny caverns, or spaces, inside these cylinders fill with blood. Blood floods the penis through two central arteries, which run through the corpora cavernosa and branch off into smaller arteries. The amount of blood in the penis increases sixfold during an erection. The blood filling the corpora cavernosa compresses and then closes off the openings to the veins that normally drain blood away from the penis (see Figure 4). In essence, the blood becomes trapped, maintaining the erection.
How does testosterone come into play? Scientists aren’t completely sure. Pumped out mainly by the testes and in small amounts by the adrenal glands, levels of this male sex hormone decline with age. Current thinking is that testosterone stokes the engine of sex drive, helping to trigger and sustain an erection. Some estimates suggest one to two out of 10 men with erectile dysfunction (ED) also have hormonal abnormalities. Yet a man with virtually no testosterone may still be able to have an erection. And a large study of older men, published in The Journal of Urology in 2006, reported no connection between ED and levels of various sex hormones, with the exception of above-normal levels of luteinizing hormone (see “Hormonal disorders”). So, despite all the talk about hormones and virility, doctors currently don’t know exactly what role sex hormones play in normal libido or the ability to have erections.
Figure 4: In working order
When a man is sexually stimulated, chemical signals from the brain cause the penile arteries to widen, allowing more blood to enter the erectile bodies known as the corpora cavernosa. The tissues swell with blood, causing an erection. At the same time, the blood-engorged tissues compress the veins, keeping blood in the penis and maintaining the erection.
Obviously, an erection isn’t permanent. Some signal — usually an orgasm, but possibly a distraction, interruption, or even cold temperature — brings an erection to an end. This process, called detumescence, occurs when the chemical messengers that started and maintained the erection stop being produced, and other chemicals, such as the enzyme phosphodiesterase 5 (PDE5), destroy the remaining messengers. Blood seeps out of the passages in the corpora cavernosa. Once this happens, the veins in the penis begin to open up again and the blood drains out. The trickle becomes a gush, and the penis returns to its limp, or flaccid, state.
It’s usually difficult for a man to get another erection right away. The length of the interval between erections varies, depending on a man’s age, his health, and whether he is sexually active on a regular basis. A young, sexually active man in good health may be able to get an erection after just a few minutes, whereas a man in his 50s or older may have to wait 24 hours. One reason may be that nerve function slows with age.
Indeed, erections may work on a use-it-or-lose-it principle. Some research suggests that when the penis is flaccid for long periods of time — and therefore deprived of a lot of oxygen-rich blood — the low oxygen level causes some muscle cells to lose their flexibility and gradually change into something akin to scar tissue. This scar tissue seems to interfere with the penis’s ability to expand when it’s filled with blood.