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8 Things You Didnt Know About Your Penis

Sensitivity, pleasure, size, and other surprising facts.

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You need to have erections regularly to keep yourpenisin shape. It has to be essentially exercised, says Tobias Kohler, MD, assistant professor of urology at Southern Illinois University School of Medicine.

To maintain a healthy tone, the smooth muscle of thepenismust be periodically enriched with oxygen by the rush ofbloodthat engorges the penis and makes it erect, Kohler says.

If guys are physically able to get erect, but never have erections during the day — maybe they find themselves in very un-erotic circumstances for a long time — they neednt worry. Thebrainhas an automatic penis maintenance function built in.

Impulses from thebraincause erections during thedreamingphase ofsleep, called the REM phase. It doesnt matter if youre having a hotor a zombie apocalypse nightmare — your penis gets hard during that period of thesleepcycle.

But some men are physically unable to get erections, such as those whove suffered trauma to the nerves involved or who have nerve orbloodvessel damage caused bydiabetes.

If they dont do anything to maintain normal erections, they will get shortening of the penis, Kohler says. Without regular erections, penile tissue can become less elastic and shrink, making the penis 1-2 centimeters shorter.

A device like a vacuum pump, which forces the penis to swell with blood, can help men with physical erection problems maintain a healthy penis, Kohler says.

Among men, there is no consistent relationship between the size of the flaccid penis and its full erect length.

In one study of 80 men, researchers found that increases from flaccid to erect lengths ranged widely, from less than a quarter-inch to 3.5 inches longer.

Whatever the clinical significance of these data may be, the locker-room significance is considerable. You cant assume that a dude with a big, limp penis gets much bigger with an erection. And the guy whose penis looks tiny might get a surprisingly big erection.

An analysis of more than a thousand measurements taken bysexresearcher Alfred Kinsey shows that shorter flaccid penises tend to gain about twice as much length as longer flaccid penises.

A penis that doesnt gain much length with an erection has become known as a show-er, and a penis that gains a lot is said to be a grower. These are not medical terms, and there arent scientifically established thresholds for whats a show-er or a grower.

Kinseys data suggest that most penises arent extreme show-ers or growers. About 12% of penises gained one-third or less of their total length with an erection, and about 7% doubled in length when erect.

Many men consider the underside of the glans (head) of the penis and the underside of the shaft to be most sensitive to sexual pleasure.

Researchers asked 81healthy mento rate the erotic sensitivity of different areas of their bodies, including not only the penis but also zones such as the scrotum,anus, nipples, and neck.

The underside of the glans and underside of the shaft had the highest sensitivity rating for a significant majority of men, followed by the upper side of the glans, left and right sides of the glans, sides of the penis, upper side of the shaft, and foreskin (for the minority of men who were uncircumcised). The study findings were reported in theBritish Journal of Urology Internationalin 2009.

Studies show that the penis steadily loses sensitivity as men age though its hard to say exactly by how much. Thats because different researchers have used different ways to stimulate the penis and measure sensitivity.

In general, the sensitivity of the penis is gauged by the least amount of stimulation a man is able to feel. That is called the sensory threshold.

Despite differences between studies, the data show a clear trend when taken together. From age 25 on, sensitivity starts to decline. The sharpest decline in sensitivity is seen between age 65 and 75.

Whats less clear is whether men really notice a loss of sensitivity as they age.

Kohler says that if they are aware of it, his patients seldom mention it.

It is a super-rare complaint, he says. On the other hand, difficulty with erections and difficulty achieving ejaculation are much more common.

Vibrators arent only for women. They work on the penis, too. In fact, vibration is so effective on the penis that often men with spinal cord injuries can ejaculate with the aid of a special medical vibrator. For this kind of treatment, the vibrator is usually held against the underside of the head of the penis.

Medical-grade vibrators arent necessarily more powerful, Kohler says. The vibrators are tuned to stimulate parts of thenervous systeminvolved in ejaculation. They work at frequencies or amplitudes that are more specific to the [nerve] pathways.

Most men dont need a medical vibrator to trigger an orgasm. Kohler says when patients see him about delayed ejaculation — difficulty reaching orgasm — he suggests they try a store-bought vibrating personal massager.

Although vibrators often help men with ejaculation problems, you dont have to have any kind of medical condition to use one. You could do it just for fun.

Most guys would be proud to know that their penis is twice as long as they think it is, Kohler says.

Thats because half the length of the penis is inside your body. Just like you dont see all of a big oak tree above ground, you dont see the root of your penis tucked up inside your pelvis and attached to your pubic bone.

As seen in anMRIpicture, an erect penis is shaped like a boomerang.

Theskinof your penis is home to a diverse community of bacteria.

Lance Price, PhD, and Cindy Liu, PhD, researchers at the Translational Genomics Research Institute, in Flagstaff, Ariz., used genetic tests to identify the bacteria found on mens penises. Their study showed there were a total of 42 unique kinds of bacteria inhabiting the skin of the penis.

We see that the human body is essentially an ecosystem, Price says.

But uncircumcised and circumcised penises dont have the same variety and abundance of bacteria, the study showed. The researchers first analyzed samples from the penises of 12 men who were planning to get circumcised. Samples were taken and analyzed again after the men were circumcised.

Aftercircumcision, there were fewer kinds of bacteria on the mens penises. Many of the kinds of bacteria found to be less common or absent aftercircumcisionwere anaerobic — meaning that they dont need oxygen to grow.

The inner fold of the foreskin is a mucous membrane, like the inside of a persons eyelids. Price says that certain anaerobic bacteria thrive in that environment but not ondry skin.

I liken it to clear-cutting a forest, Price says. Youre going to get a lot more sunlight, and youre going to drastically change the environment.

The study was done in Uganda, and all of the men studied were Ugandan.

Liu says that she would expect to see some variation in the kinds of bacteria found on men in other parts of the world. I think there is certainly variety even among the Ugandan men themselves, she says.

But the researchers are less interested in surveying the penile bacteria of the world than in understanding changes brought about bycircumcision.

Their research could help explain why circumcision has been linked to a lower risk of gettingHIV. One theory is the anaerobic bacteria may prompt the immune system to respond in a way that makes cells more vulnerable toHIVinfection.

Worldwide, approximately 30% of males aged 15 and older are circumcised, according to a 2007 report from the World Health Organization (WHO) and UNAIDS.

Rates vary greatly depending upon religion and nationality, the report states. Almost all Jewish and Muslim males in the world have circumcised penises, and together they account for almost 70% of all circumcised males globally.

Some research shows that there may be health benefits from circumcision. For instance, circumcised men may be less likely to passsexually transmitted diseasesto their female partners or to develop penilecancer.

A 2012 policy statement by the American Academy of Pediatrics (AAP) affirms, Evaluation of current evidence indicates that the health benefits of newborn male circumcision outweigh the risks and that the procedures benefits justify access to this procedure for families who choose it. Specific benefits identified included prevention of urinary tract infections, penile cancer, and transmission of some sexually transmitted infections, including HIV. This statement has also been endorsed by the American College of Obstetricians and Gynecologists.

In recent years, several studies have shown that circumcised men are less likely to be infected with HIV. The WHO and UNAIDS now recommend male circumcision as an HIV prevention measure. There is compelling evidence that male circumcision reduces the risk of heterosexually acquired HIV infection in men by approximately 60%, the WHO states.

Tobias Kohler, MD, MPH, assistant professor of urology, Southern Illinois University School of Medicine, Springfield, Ill.

Lance Price, PhD, Cindy Liu, PhD, Translational Genomics Research Institute, Flagstaff, Ariz.

Hirshkowitz, M.Sleep Medicine Reviews, August 2005.

Wessells, H.Urology, September 1996.

Jamison, P.Journal of Sex Research, January 1988.

Wylie, K.British Journal of Urology International, June 2007.

Schober, J.British Journal of Urology International, April 2009.

Brackett, N.Human Reproduction Update, May-June 1999.

Faix, A.Journal of Sex and Marital Therapy, 2002.

World Health Organization and UNAIDS: Male circumcision: Global trends and determinants of prevalence, safety and acceptability, February 2007.

American Academy of Pediatrics Task Force on Circumcision,Pediatrics, March 1, 1999.

Maeda, J. HCUP Statistical Brief 126, February 2012, Agency for Healthcare Research and Quality, Rockville, Md.

CDC: Male Circumcision and Risk for HIV Transmission and Other Health Conditions: Implications for the United States.

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