11/19/10 - viagra (Warning: May Not Be Suitable for All Audiences)

In today's excerpt - from the New York Times best-seller, Bonk, an explanation of erectile dysfunction, along with a brief mention of both Leonardo da Vinci and erections in your nose:

"Erections are all about blood. Blood is the backbone of a stiff penis. Though it was a long time before anyone figured this out. In the Middle Ages, the erect male member was thought to be filled with pressurized air, a miniature skin blimp. It was Leonardo da Vinci who made the breakthrough. Cadavers available for anatomy study back then were typically those of executed murderers. Because they'd been hanged, the dead criminals had erections, and because Leonardo was dissecting them, he noticed that their penises were, in his very own words, 'full of a large quantity of blood.'

"The blood resides in a pair of cylindrical chambers—the corpora cavernosa—which lie side by side like a diver's tanks. The chambers are filled with smooth-muscle erectile tissue, full of thousands of tiny hollow spaces, like a sponge.  When the smooth-muscle tissue relaxes—which it does at the behest of an enzyme activated when the brain perceives a sexual stimulus—it expands. (Smooth muscle, unlike the striated muscles of your arms and legs, is operated by the autonomic nervous system; this is why men can't simply will themselves erect—or unerect.) The relaxation of the erectile tissue allows blood to rush in and fill out the spongy hollows. Drugs like Viagra enhance the erection process by knocking out a substance nicknamed PDE5, which inhibits its smooth-muscle relaxation. They inhibit the inhibitor. (Thus, they're called PDE5 inhibitors.)

"So now we have achieved, in the parlance of ED experts, an erection. It is a respectable achievement, but it is not enough. An erection, like a motorcycle or a lawn, must also be maintained. The blood that has filled the two erection chambers* must be trapped there, otherwise the erection wilts. This is tricky, as the chambers are equipped with drainage veins along their surface. What keeps the blood from leaking out via these veins? The miracle of passive venous occlusion. (Stay with me here.) These drainage veins lie outside the erection chambers but inside the stiff outer membrane (called the tunica) that protects the erectile tissue. When the chambers expand with blood, they slam up against the tunica—which also expands, but not as much—and this pressure squeezes shut the veins caught in between. If all goes well, the blood stays trapped until a postorgasm chemical messenger tells the smooth-muscle tissue to stop relaxing. When a man is impotent, very often it's because the erectile tissue isn't expanding as vigorously as it needs to squeeze shut the veins, and some of the blood seeps out.

"The result: 'Like a tire! Flat!' [explains] Dr. Hsu.

"*Actually, there's a third, that runs beneath these two, but it's a lesser player and we're going to ignore it. Likewise, we are going to ignore the erectile tissue in the lining of the nose—which does, very occasionally, expand when its owner is sexually aroused. It too is made erect by increased blood flow. Nasal congestion is an erection inside your nose."



Mary Roach


Bonk: The Coupling of Science and Sex


W.W. Norton & Company, Inc.


Copyright 2008 by Mary Roach


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