How do erections normally occur

In order to understand how an erection occurs, one must first learn a little about the anatomy of the penis. The penis may look like one simple tube, but it is actually comprised of three cylinders. There are two on the top of the penis called the corpora cavernosa (a Latin phrase meaning, roughly, "bodies composed of hollows or caves") and one on the underside of the penis, the corpus spongiosum ("sponge-like body") (Figure 28). The tip of the penis, called the glans, is part of the corpus spongiosum. The corpora cavernosa are surrounded by a fibroelastic layer of tissue, the tunica albuginea (literally, "white coat," referring to fact that the tunica albuginea is a thick white membrane wrapped around the

Bulbourethral y gland Coronal section through shaft of penis

Bulbourethral y gland Coronal section through shaft of penis

Corpus cavernosum Corpus spongiosum Urethra

Corpora cavernosa Tunica albuginea of corpora cavernosa: Outer longitudinal layer and inner circular layer

Corpus spongiosum

Urethra

Corpora cavernosa Tunica albuginea of corpora cavernosa: Outer longitudinal layer and inner circular layer

Corpus spongiosum

Urethra

Trabecular tissue

Sinusoids

Bucks fascia

Skin Superficial fascia

Trabecular tissue

Sinusoids

Bucks fascia

Skin Superficial fascia

Figure 28 Anatomy of the penis.

Reprinted with permission from Br J Urollntl 2001; 88(Suppl 3):3—10.

corpora cavernosa like a cloak). The two corpora cavernosa contain numerous compartments that are filled with blood during sexual excitement, which is what makes the penis become erect. The corpus spongiosum contains the urethra, the tube that one urinates through, and it is not involved in the erectile process.

The two corpora cavernosa and the corpus spongiosum each have an artery that supplies it. The artery to each corpus cavernosum runs through its center (Figure 29). The two corpora cavernosa communicate in the middle of the penis, thus allowing blood from one corpus cavernosum to flow into the other. The veins that drain the penis are also different for the corpus spongiosum and the corpora cavernosa. The veins that drain the corpora cavernosa, unlike the arteries, run along the outer edge of the corpora cavernosa, just underneath the tunica albuginea (Figure 29).

The reason why these veins and arteries are important is that when a man becomes aroused, his brain and the r n i

Artery

A blood vessel that carries oxygenated blood from the heart to other parts of the body.

Vein

A blood vessel in the body that carries deoxygenated blood from the tissues back to the heart.

Flaccid state

Flaccid state

Tunica albuginea

Emissary vein

Erect state

Tunica albuginea

Emissary vein

Erect state

Increased blood flow through helicine arteries to sinusoids

Cavernosal artery dilates

Increased blood flow through helicine arteries to sinusoids

Cavernosal artery dilates

Compression of emissary veins leads to decreased venous outflow

Tunica albuginea stretches to maximum capacity Sinusoid fills with blood

Compression of emissary veins leads to decreased venous outflow

Tunica albuginea stretches to maximum capacity Sinusoid fills with blood

Figure 29 Mechanisms of normal erectile response.

nerves in his pelvis release chemicals that increase blood flow into the penis. The corpora cavernosa are similar to a sponge: just as a sponge absorbs liquids into its air spaces and distends when submerged, the corpora cavernosa have hollow spaces, or sinusoids, that distend with blood when sexual excitement causes increased blood flow to the penis. As the sinusoids fill with blood and distend, they compress the veins against the tunica albuginea. It is this compression of the veins that prevents blood from draining out of the penis, which promotes full rigidity and maintenance of the rigidity (Figure 29).

Nerve

A cordlike structure composed of a collection of nerve fibers that conveys impulses between a part of the central nervous system and some other region of the body.

Pelvis

The part of the body that is framed by the hip bones.

You might be surprised to learn that erections aren't simply something that happens in the penis. For an erection to occur, there must be proper functioning of many physical structures and systems: the brain, certain nerves in the pelvis, and the arteries and veins that supply the penis. When a man is aroused, the brain tells nerves in the pelvis to release neurotransmitters, which in turn stimulate the blood vessels in the penis to open up and the smooth muscle in the corpora cavernosa to relax to increase blood flow into the penis. After sexual performance is completed, the brain releases other chemicals that tell the arteries in the penis to constrict, thus decreasing blood flow to the penis and allowing the veins to drain the blood out of the penis. These chemicals that cause constriction of smooth muscle may also be released during times of stress and may adversely affect erectile function.

Now that you know that the erectile process is a neu-rovascular event, then it becomes clear that any disease process that affects the brain, the nerves in the pelvis, the arteries to and within the penis, the veins in the penis, the tunica albuginea, or the erectile tissue within the corpora cavernosa may affect erectile function.

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