01 November 2009

How the penis works

If you feel more at home under the hood of a car than in front of a diagram of the anatomy of the male private parts, you might want to read this.


Whether they refer to them as their “crown jewels”, or their “private part”, or whether they prefer not to talk about them at all, most men know precious little about their reproductive organs - the penis and its associated bits and bobs. The majority of us would probably feel more at home under the hood of a car than in front of a diagram of the anatomy of the male privates.

The general plumbing in your groin turns out to be much less complicated than you might have expected and the basics are really quite easy to grasp.

The basic layout

The penis consists of three main parts: the base, where it is attached to your lower abdomen, the shaft, which forms the elongate bit and the head, which is also referred to as the glans. Although it is abundantly supplied with nerve endings, it may come as a surprise that the human penis doesn’t contain any bones. Not so much a “boner” then after all...

The shaft contains two elongate, cylindrical tubes, known as the corpora cavernosa, which consist of spongy tissue and run the length of either side of the penis. During an erection, the corpora, which are encased in a tough sheet called the tunica, fill with blood and expand.

A third cylinder of tissue, the corpus spongiosum, runs through the shaft between the two corpora cavernosa. It contains the urethra, the tiny tube through which both urine and semen are expelled from the body. Towards the tip of the penis, the corpus spongiosum expands to form the helmet-shaped glans, which is covered by the foreskin in uncircumcised boys and men.

The urinary tract

One of the two main functions of the penis is to rid the body of urine. You can think of urine as the body’s liquid waste product, which is filtered out of the blood by the kidneys. Two tubes, called ureters, deliver it from the kidneys to the bladder, where it is stored until it is expelled from the body via the urethra, which connects the bladder to the penis.

The sperm factory

The testes or testicles - balls to you and me - manufacture the male sex hormone testosterone, which is responsible for so-called secondary male sex characteristics, including facial and pubic hair, muscle development and thickened vocal chords.

The testes are also an amazing production facility that pumps out sperm (i.e. male reproductive cells) in vast numbers. About 100 million each and every day from the time you’re a teenager into old age. They contain a mass of tiny, coiled tubes that are lined with sperm-producing cells.

The testes are housed in the scrotum, which, while not the most attractive part of the male anatomy, is very effective at keeping them at a cool and steady 35-36 degrees C, one to two degrees below the rest of the body. In case you’ve ever wondered, one of them usually hangs a little lower than the other. No need for alarm. It’s perfectly normal.

Freshly made sperm travels from each testis to a coiled tube on its outer surface known as the epididymis. Here, like good wine in an oak cask, the sperm cells mature for about 20 days to six weeks.

During an orgasm, mature sperm cells - some 400 million of the little buggers - are rapidly expelled from the epididymis through a long duct called the vas deferens. The two vasa deferentia (yes, that’s the plural form of vas deferens) from the testicles are connected to the urethra (remember, that’s the tube that runs through the penis and also caries urine to the outside) through which the sperm is ejaculated.

The prostate gland & Co.

Before it reaches the outside world, however, the sperm is first mixed with fluids from the prostate gland and the seminal vesicles, creating that sticky concoction known to all of us as semen.

The prostate is a walnut-sized gland located just below the bladder and in front of the rectum. Its contribution to the semen, approximately 15 to 30% by volume, is a thin, milky liquid that improves the survival and motility (i.e. the ability to move spontaneously) of the sperm cells.

The seminal vesicles are two 5 to 7 centimetre long, elongated, sac-like glands positioned just above the prostate that secret a fluid which makes up about 60% of the semen, provides energy and nutrition for the sperm cells and aids them in the process of fertilisation.

Cleaning the pipes

If you’ve been reading this carefully, you’ll have noticed that the urethra forms the passageway through which both urine and semen are excreted from the body. There’s no need to worry, though. When the penis is erect, a tiny valve closes the urethra off from the bladder, making it impossible for urine to mix with semen. This also explains why it is rather difficult to urinate with a hard-on!

In addition, during sexual arousal and just before ejaculation, two pea-sized glands at the base of the penis, called the bulbourethral or Cowper’s glands, inject a little bit of fluid into the urethra that flushes out and neutralises any remaining urine and also helps with lubrication.

More penis info:

Penis 101
How an erection happens
What's wrong with my penis?
Penis resources
Penis size per country
The lowdown on the penis
The lowdown on your testicles
Your foreskin
Erectile Dysfunction (ED)


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