The penis has a dual role as a urinary organ allowing directed urination that permits men to stand to urinate, and a sexual and reproductive organ that, when erect, allows the rigid penis the ability to penetrate the vagina and function as a conduit for release of semen into the vagina. No other organ in the body demonstrates such great versatility in terms of the physical changes between its “inactive” versus “active” states.
The penis consists of three cylinders: the solitary corpus spongiosa contains the urethra (the channel that conducts urine from the bladder); the other paired cylinders are called the corpora cavernosa (erectile bodies), that are anchored internally to the pubic bone and extend into the glans (head of the penis).
These corpora cavernosa communicate with each other and are enclosed in a sheath called the tunica albuginea. The corpora contain sinusoids (spongy tissue) that are endowed with a very rich blood supply. The sinusoids receive blood flow via the cavernosal arteries which are branches of the pudendal arteries. When the corpora become engorged with blood, an erection results.
The seemingly simple process of achieving an erection is actually a highly complex event requiring integrated functioning of the brain, nerves, blood vessels, and hormones.
When the penis is flaccid, there is only minimal arterial inflow, sufficient to maintain the basic nutritional demands of the penis. During this unerect state, the sinusoids are closed while the venules remain open. However, when the cavernosal nerve is stimulated by sexual activity, the smooth muscle of both the cavernosal arteries and the corporal bodies relaxes, allowing blood to fill the corpora. Furthermore, the swelling of the corpora obstructs the venous outflow to maintain the erection. The pudendal nerve provides the nerve supply to the ischiocavernosus and bulbocavernosus muscles that enhance penile rigidity, engorgement of the glans, and allow ejaculation and orgasm when these muscles contract rhythmically. So, for an erection to occur, 3 events need to happen—an increase in arterial flow to the corporal bodies, relaxation of smooth muscle, and a decrease in venous outflow.
What is actually happening on a chemical basis when one is involved in a sexually stimulating situation? The control center for erections is the brain, where sensations of sexual arousal are experienced. The brain sends signals to the cavernosal nerves, which are also stimulated by direct sensory contact, such as foreplay or the act of sexual intercourse itself. During such sexual stimulation, the cavernosal nerves release a neurotransmitter, nitric oxide, which causes an increase of a chemical messenger known as cGMP (cyclic guanosine monophosphate) within the smooth muscle of the corporal bodies. This causes relaxation of the smooth muscle and enlarges the sinusoids, providing the space for the increased blood flow, causing penile tumescence or turgidity (lengthening and widening of the penis). This engorgement with blood causes compression of the veins directly under the tunica albuginea, trapping the blood within the penis. After ejaculation, an enzyme called PDE (phosphodiesterase) is released—this degrades cGMP resulting in a return to the flaccid state by a reversal of the aforementioned mechanisms. Viagra, Cialis, and Levitra work by inhibiting PDE.