You and Your Penis!

The Male Reproductive System - How It Works

You know about your penis, of course, since it sits outside your body, very visible, and hopefully very pleasing to you. You also know about your testes, or testicles, hanging in their scrotum in front of you; your balls, some might say, are what make you a man, not your penis. But there is much more to the male reproductive system than the penis and testicles.

The internal organs include the testes, epididymis, vas deferens, seminal vesicles, prostate gland, and various other glands such as the Cowper's Glands which produce pre-cum or pre-ejaculatory fluid.

We shall start with the testes. These two organs produce sperm and testosterone. The hormone testosterone is essential for a boy's normal sexual development at puberty, and for the development of his secondary sexual characteristics, including body and facial hair, penile growth, muscular development, libido, and his ability to ejaculate seminal fluid. The sperm which are produced in the testes pass into the epididymis for storage; before ejaculation they are moved into the vas deferens, two tubes that link the testes to, first, the prostate gland and, second, the urethra which runs through the centre of the penis to the outside world.

Just before ejaculation, sperm are mixed with the secretions produced by the seminal vesicles and prostate gland. The prostate gland contributes about a third of the ejaculatory fluid; the sperm comprise a very small fraction of the ejaculate. After vasectomy, the amount of ejaculate a man produces does not change much. 

The prostate gland surrounds the urethra where it emerges from the bladder and is often the cause of urinary difficulties in older men because it enlarges or becomes cancerous, thereby blocking the urethra. There is a mechanism which prevents urine emerging from the bladder at the moment of orgasm. When the muscular contractions of ejaculation begin, the semen is shot along the urethra to the end of the penis, and the seminal fluid is then ejaculated.

The penis does not contain muscle tissue which can be enlarged with any kind of exercise, though enlargement sites do offer stretching exercises which can apparently lead to some kind of increase in size (along with damaged erectile tissue). Indeed, most of the volume of the penis is occupied by two cylinders which contain spongy tissue protected by an elastic membrane with a fibrous covering. As you no doubt know, when you become aroused, these cylinders fill with blood and your penis swells by up to three times its resting volume. As the cylinders fill with blood, the elastic sheath around the cylinders compresses the veins which would normally allow blood to flow out of the penis; since it is trapped, the penis remains hard and an erection continues for as long as sexual arousal continues.

Different parts of the penile shaft have differing levels of sensitivity to stimulation, though most men would probably agree that the head of the penis is the most sensitive part, at least in terms of sexual pleasure. This is after all the part which receives most stimulation during intercourse or masturbation, and is the part which is thought to trigger the ejaculatory reflex. Having said that, the scrotum is very sensitive to stimulation, and gentle touch on it plays a part in most couple's foreplay.

Here is a plan of the male reproductive system.

The penis

The penis varies between men: but the basic form of the parts which make it up are similar from man to man. The shaft ends in a round or conical "helmet" called the glans. Around the bottom edge of the glans is the exquisitely sensitive coronal rim which contains a lot of nerve endings; the same is true of the frenulum, where the foreskin is attached to the penile shaft. This is actually one of the most sensitive parts of the penis. Obviously when a man is circumcised there will be no foreskin at the end of his penis. Right at the top of the glans is the opening of the urethra, the place form which urine and semen emerge during urination and ejaculation respectively.

The internal structure of the penis can be seen in the diagram below. You can see the spongy bodies, two at the top of the penis and one underneath, known as the Corpus Cavernosum (plural Corpora Cavernosa) and Corpus Spongiosum respectively. As we previously described, these spongy tissues fill with blood during an erection.

The corpus spongiosum is smaller and thinner than the corpora cavernosa, partly because the urethra runs through it to the end of the penis. At the tip of the end of the penis, the corpus spongiosum enlarges into the glans. Various arteries and veins, which again you can see illustrated on the diagrams below, run through the erectile tissue, supplying the tissues of the penis and the erectile mechanism itself with blood. An erection is produced when more blood comes into the penis than leaves it, so all of these blood vessels must be free of cholesterol deposits and clear for the penis to become hard and erect.

As we implied above, one crucial part of the erectile mechanism is the membrane around each spongy body. This tissue, the fascia called the tunica albuginea, surrounds the three spongy bodies, which in turn is surrounded by tissue known as Buck's fascia. The veins and arteries of the penis lie in this layer of tissue which is in turn surrounded by the superficial or Colles fascia.

The scrotum

The scrotum is the supporting structure which holds the testicles; it's made up of loose skin, fascia and muscle. The bag of skin which makes up most of the volume of the scrotum is divided into two by a fold of skin called the raphe - this reflects the internal division into two compartments, one for each testicles. The scrotum can contract under conditions of cold or fear, or just prior to ejaculation, a fact which demonstrates the activity of the dartos muscle in the scrotum, which is contiguous with the smooth subcutaneous muscles of the body's abdominal wall. The dartos muscle is more relaxed when the scrotum is warm and the scrotum may hang much lower - this allows the testes to remain cooler than the normal body temperature, a fact which is needed for sperm production. The left testis normally hangs a bit lower in the scrotum than the right one: it's also normal for one testis to be somewhat larger than the other.

More on the various parts of the male reproductive system

Next: The Male Sexual Response

D[ How do you see your penis? ] [ The penis and testicles or testes ] [ Male sexual response ] [ The significance of your erection ] [ The flaccid penis ] [ Your penis and your life ] [ Using your penis in bed ] [ Fantasies ] [ A Typical Sexual Fantasy ]