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Mechanism of Erection

The penis contains two chambers called the corpora cavernosa, which run the length of the organ (see figure below). A spongy tissue fills the chambers. The corpora cavernosa are surrounded by a membrane, called the tunica albuginea. The spongy tissue contains smooth muscles, fibrous tissues, spaces, veins, and arteries. The urethra, which is the channel for urine and ejaculate, runs along the underside of the corpora cavernosa.

Erection begins with sensory or mental stimulation, or both. When a man becomes aroused through visual or physical stimulation the brain sends signals to the genital area that triggers the release of nitric oxide in the penis. This event is the catalyst that begins a chain of events leading to the erection. Nitric oxide in the penis activates the enzyme guanylate cyclase. This enzyme causes increased levels of cyclic guanosine monophosphate (cGMP). When levels of cGMP are increased in the penis this causes a smooth muscle relaxation in the pelvis and corpora cavernosa and allows increased blood flow into the corpora cavernosa.

As the corpora cavernosa fills with blood it begins to expand due to the pressure created by blood, making the penis expand. And as it expands the arteries that normally carry blood out of the penis become restricted and are unable to carry blood out faster than blood is coming in. The tunica albuginea helps trap the blood in the corpora cavernosa. With more blood coming in and less going out the corpora cavernosa continues to expand and harden, thus resulting in an erection. When muscles in the penis contract to stop the inflow of blood and open outflow channels, erection is reversed.

Erectile Dysfunction
Erectile dysfunction is the repeated inability to get or keep an erection firm enough for sexual intercourse. Starting at midlife, the vessels of the penis begin to stiffen and become less pliable, making it difficult for the blood to easily flow through them. Current estimates suggest that more than half of all men in the United States over age 40 experience some erectile problems, according to the Massachusetts Male Aging Study, published in the journal Urology.

ED is treatable at any age. Awareness of this fact has been growing.  Since an erection requires a precise sequence of events, ED can occur when any of the events is disrupted. The sequence includes nerve impulses in the brain, spinal column, and area around the penis, and response in muscles, fibrous tissues, veins, and arteries in and near the corpora cavernosa. Damage to nerves, arteries, smooth muscles, and fibrous tissues, often as a result of disease, is the most common cause of ED. Diseases--such as diabetes, kidney disease, chronic alcoholism, multiple sclerosis, atherosclerosis, vascular disease, and neurologic disease--account for about 70 percent of ED cases. Between 35 and 50 percent of men with diabetes experience ED. Also, surgery (especially radical prostate surgery for cancer) can injure nerves and arteries near the penis, causing ED. Injury to the penis, spinal cord, prostate, bladder, and pelvis can lead to ED by harming nerves, smooth muscles, arteries, and fibrous tissues of the corpora cavernosa.

In addition, many common medicines--blood pressure drugs, antihistamines, antidepressants, tranquilizers, appetite suppressants, and cimetidine (an ulcer drug)--can produce ED as a side effect. Experts believe that psychological factors such as stress, anxiety, guilt, depression, low self-esteem, and fear of sexual failure cause 10 to 20 percent of ED cases. Men with a physical cause for ED frequently experience the same sort of psychological reactions (stress, anxiety, guilt, depression). Other causes are smoking, which affects blood flow in veins and arteries, and hormonal abnormalities, such as not enough testosterone.

One-third of the men surveyed in a study appearing in the August 5 issue of the Annals of Internal Medicine reported having problems with an erection in the prior three months. While the incidence of erectile dysfunction (ED) increased about 5 percent each year for all men, the problem was notably less common among men leading healthy lifestyles.  Being a vigorous exerciser and adding other healthy lifestyle factors such as eating nine servings of fruits and vegetables a day,  not smoking, staying lean and not drinking had the effect of adding 10 years to a man's sexual status. Men who did the equivalent of three hours of running or more each week or playing five hours of singles tennis reported a 30 percent lower risk of ED, compared with those who did little or no exercise.  Exercise seems to benefit the small arteries that control erections, much as exercise benefits other arteries, such as those that feed the heart. Thus, what happens to the penis may be an early warning of what could happen to the heart, such as a heart attack. Watching more than 20 hours of television each week, consumption of alcohol, smoking and being overweight were associated with higher levels of ED, as was having diabetes, previous stroke or taking antidepressants or beta-blockers. The risk factors for ED were about the same as those for heart disease. 

Drugs for treating ED can be taken orally, injected directly into the penis, or inserted into the urethra at the tip of the penis. In March 1998, the Food and Drug Administration approved Viagra, the first pill to treat ED. Taken an hour before sexual activity, Viagra works by enhancing the effects of nitric oxide, a chemical that relaxes smooth muscles in the penis during sexual stimulation and allows increased blood flow. While Viagra improves the response to sexual stimulation, it does not trigger an automatic erection as injections do. The recommended dose is 50 mg, and the physician may adjust this dose to 100 mg or 25 mg, depending on the patient. The drug should not be used more than once a day. Men who take nitrate-based drugs such as nitroglycerin for heart problems should not use Viagra because the combination can cause a sudden drop in blood pressure.

In August 2003, the Food and Drug Administration has approved Levitra (from the word levitate). Levitra is a new drug made by Bayer and Glaxosmithkline that treats erectile dysfunction in men. When taken prior to sexual activity, Levitra helps a man achieve stronger and longer lasting erections for sexual intercourse. It contains vardenafil HCl, a drug in the same family as Viagra, for erection. Levitra comes with the same warnings as Viagra. It should not be taken by men who already take a nitrate-containing heart medication or alpha blocker drugs for high blood pressure or enlarged prostate. Men who've recently had a heart attack or stroke, men with very low or uncontrolled high blood pressure, and men with the rare heart condition called QT prolongation also should not take Levitra. The FDA suggests that all men undergo a physical exam before starting on Levitra. The main side effects in healthy men were headache, flushing, a stuffy nose and, rarely, dizziness.

Additional oral medicines may soon be available to treat ED.  Cialis is being tested for safety and effectiveness. Cialis is a PDE5 inhibitor developed for the treatment of erectile dysfunction. Cialis is already available by prescription in approximately 40 countries worldwide including Europe, Brazil, Australia, New Zealand, and Singapore.Cialis is not yet approved for sale in the United States and is currently under review by the U.S. Food and Drug Administration ("FDA"). Lilly ICOS continues to expect a U.S. regulatory decision regarding Cialis late in 2003.  Another drug being tested, Uprima, works on the brain and nervous system to trigger an erection. Uprima is an Abbott Laboratories product.

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Copyright Vepachedu Educational Foundation Inc., 2003.  All rights reserved.  All information is intended for your general knowledge only and is not a substitute for medical advice or treatment for special medical conditions or any specific health issues or starting a new fitness regimen. Please read disclaimer.

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