Do you know that erection of penis is only due to accumulation of blood?

Penile erection

Erection of the penis of a male partner during a sexual act is one of the most crucial parts. Actual intercourse starts with the erection of the penis and end with an ejaculation of semen. Although penis erects and becomes hard (in any time, including during the sex), there is no muscles or bones present in the penis. The whole phenomenon is the action of blood and blood vessels.

Structure of a penis

To understand this, we need to know the structure of a penis in short. Along the whole length of a penis, beside the urethra (the passage for urine or semen), there are two tube-like structures side-by-side on the back of the penis which is called corpora cavernosa (singular is corpus cavernosum). There is one artery in the centre of each corpus cavernosum along the long axis. A corpus is not just a solid tube, rather it is divided into numerous small spaces or cavities separated by structures called septa.  In a similar fashion, blood vessels (arteries) gradually divide into smaller branches called arterioles, run within the septa and finally enter into each individual spaces. There are other types of blood vessels called veins, which run along the side of arteries but in opposite direction. 

Arteries in a corpus cavernosum carry blood into space, from outside. On the other hand, veins in the corpus cavernosum drain away blood from the spaces to outside.

The two nervous systems:

Before learning the actual mechanism of the erection, we need to know a few things about our nervous system. In our nervous system, we have two distinct types of nerves or systems which are completely automatic and beyond our voluntary control. One is the sympathetic nervous system and another one is the parasympathetic nervous system. They play a vital role in erection of the penis.

What actually happens during an erection?

When a male partner, during a sexual act, becomes excited, his parasympathetic system is activated first. This causes dilation/widening of arteries and arterioles, and closure of the veins i.e inflow is open and outflow is closed. Both of the tubes (corpora cavernosa) fill gradually with blood and erection process gradually start. When all spaces are filled with blood, penis erects completely and becomes hard or strong.

When the event is close to ending, the parasympathetic nervous system starts relaxing. Just the reverse procedure takes place. Arteries and arterioles start closing and veins start opening i.e, the outflow is now open and inflow close. So cavernous spaces become empty again and the penis becomes unerect.