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.

A few words on the embryology of our sex organs (the science of our anatomical development before we are born): You probably never realized that the male and female genitals are remarkably similar. In the first few weeks of existence as an embryo, the external genitals are the same— consisting of a genital tubercle (a midline swelling), urogenital folds (two vertically-oriented folds of tissue below the genital tubercle), and labioscrotal folds (two vertically-oriented folds outside the urogenital folds). In the presence of testosterone (the male sex hormone), the genital tubercle becomes the penile shaft and head; the urogenital folds fuse and become the urethra and part of the penile shaft; and the labio-scrotal folds fuse and become the scrotum. In the female embryo, the absence of testosterone causes the genital tubercle to become the clitoris, the urogenital folds to become the inner lips (labia minora), and the labio-scrotal folds to become the outer lips (labia majora). Essentially then, the penis and the clitoris are the same structure, as are the scrotum and outer labia.

On a functional level, sexuality is a very complex event dependent upon a number of body systems, including the endocrine system (which produces sex hormones); the central and peripheral nervous systems (which provide nerve control); and the vascular system (which conducts blood flow). While all of these systems must function in a normal, coordinated fashion, a healthy sexual response is, at its physical essence, largely about adequate blood flow to the genital and pelvic area. Increased blood flow to the genitals from sexual stimulation is what is responsible for the erect penis. In a male, blood flow to the penis is analogous to air pressure in a tire: if there is not enough air, thereby causing the tire to be improperly inflated, the tire works less optimally.