About the Penis

The penis is part of the male reproductive or sexual system, which also includes body parts like the scrotum and testicles. In order to understand how the penis works, it can help to think of it in two parts – the outside (or what you can visibly see) and the inside (or what you can’t see with the naked eye).

On the outside, the shaft of the penis is covered by a thin layer of skin, which is loose when unaroused or flaccid and in uncircumcised men folds in on itself over the ‘head’ of the penis to form the foreskin. This head is technically called the ‘glans penis’ , an area which is highly sensitive thanks to the number of nerves it contains. Round in shape when flaccid, when its erect, the penis takes on a more triangular shape, but with slightly rounded edges.

On the inside, the penis is made up of three cylindrical masses of tissue – two of which run along the outside of the penis and together, are called the corpora cavernosa penis, and the third, called the corpus cavernosum urethrae (or the corpus spongiosum) which runs up the centre. All three are usually relatively empty of bodily fluids, but during arousal they become filled with blood, which is what supports a man’s erection.

Running through the middle of the corpus spongiosum is the urethra, a tube which comes to the end at the top of the head, or ‘glans’ of the penis, where a slit-like vertical opening can be seen. Essentially, the urethra connects the bladder and the prostate gland with the end of the penis and its role is twofold: not only does it transport urine out of the body, it’s also how semen makes its way to the end of the penis. Because of this, the urethra plays an important role in men’s sexual health.

What’s to stop urine and semen from leaving at the same time? That’s thanks to something called the internal sphincter, a muscle that’s located at the base of the bladder. When an orgasm occurs, the muscle shuts tightly to prevent urine and semen trying to ‘use’ the urethra at the same time, and to prevent sperm from passing ‘backwards’ into the bladder.