How do erections occur?

The erection of the male limb seems so simple: During sexual arousal, blood flows into the cavernous body. The hard penis enables penetration. After the ejaculation or when arousal subsides, the blood flows out of the cavernous body. The penis becomes flaccid and returns to normal. What proceeds so self-evident is regulated by a highly complex interplay. At that the brain, blood vessels and nerves play the most important roles.

Erection and sexual arousal

The first precondition for erection is the sexual arousal. This desire arises from the brain. Whether touch, memory, sight, smell or fantasy: Sexual arousal can be triggered by very different stimuli. The most important control centers are the cerebrum, limbic system and hypothalamus (an area of the diencephalon).

The cerebrum is the brain region in which sexual arousal can be controlled. It evaluates stimuli. The cerebrum is the instance that reacts, for example, to social norms such as ideals of beauty, individual preferences or gender influences.

The limbic system and the hypothalamus, on the other hand, react involuntarily and hence are not subject to will. The limbic system is a very old area of the brain that is sometimes referred to as the emotional center. The arousal stimulated by these brain areas is mainly traced back to the sexual urge and is spontaneous. One also speaks of the arousal reflex. The hypothalamus regulates the release of messenger substances that are indispensable for desire and erection.

Erection without arousal

Men know: Not every erection is the result of sexual arousal. The best example is the so-called morning wood. It occurs because the blood vessels strongly relax during sleep and the penile cavernous body can therefore fill with blood.


The process of erection

The arousal reflex generated in the brain reaches the penis via nerve tracts. On the way there and in the penis itself, messenger substances are released. This initially brings about a dilation of the cells of the smooth vascular muscles in the arteries that lead to the penis and the penile cavernous bodies. This causes an increased flow of blood into the cavernous bodies. The penis fills with blood and dilates. Due to the pressure of the cavernous bodies the veins in the penis are squeezed. Therefore, the blood cannot drain off and the erection is maintained. In a complete erection, the blood volume in the penis is about 7 to 10 times the size of a flaccid penis. Most cases of erectile problems occur because of insufficient blood supply into the penis. In many cases, this undersupply can be corrected by stent implantation of the penis arteries. Shock wave therapy for erectile dysfunction can sometimes even cure impotence permanently.

The cavernous bodies in the penis

In common usage, we mostly speak of the penile cavernous body. In fact, the male penis has three cavernous bodies.

  • The corpus cavernosum penis is the penile cavernous body that is usually referred to in common usage. This cavernous body runs from the root of the penis to the glans and forms by far the largest cavernous body in the male limb. It is of crucial importance for the erection.
  • The corpus spongiosum glandis is the cavernous body of the glans and responsible for the enlarging of the glans (glans penis) during the erection. For the erection and hardening of the limb this cavernous body plays no role.
  • Corpus spongiosum penis is the medical term for the cavernous body that surrounds the urethra at the penis shaft. This cavernous body is of no importance for an erection.

By the way, corpus spongiosum glandis and corpus spongiosum penis do not harden due to a simple reason. If the penis shaft and the glans would harden in a similar way as the corpus cavernosum penis, an ejaculation would not be possible.


Messenger substances important for an erection

All physical processes are regulated by messenger substances. In case of an erection, the arousal reflex initiates a chain reaction via the hypothalamus. Roughly simplified, it looks as follows: First of all, nitrogen oxide (NO) is released and distributed with the blood circulation. This in turn ensures that during sexual arousal, a messenger substance called cyclic guanine monophosphate (cGMP) is released by the brain in certain cells of the blood vessels. This messenger substance activates the so-called protein kinase B. It is decisively responsible for the relaxation of the smooth muscles in the blood vessels. Thus, more blood can flow through the penis arteries and cavernous bodies. The penis hardens.

How the penis becomes flaccid

The end of an erection is also controlled by messenger substances. After the ejaculation or when sexual arousal decreases, the body releases the so-called phosphodiesterase-5 (PDE5). PDE5 is an enzyme that breaks down the cyclic guanine monophosphate. Thus the vascular walls of penis arteries and cavernous bodies constrict again. Thereby, the veins open up on the penis shaft and the blood can flow out of the cavernous bodies.

The central function of the enzyme PDE5 is harnessed in the drug therapy of erectile dysfunction. Medications like Viagra or Cialis and other so-called potency-enhancing drugs are PDE5 inhibitors. They suppress the effect of PDE5 and thus ensure an improved erection. You can read more about the effect, mode of action and risks here: PDE5 inhibitors Viagra, Cialis and Co.


The question of penis length and shape

Penis length and shape are of no importance for the erectility. Independent of the size, through an erect penis flows about 7 to 10 times as much blood as through a flaccid male limb.

Average penis size

The size of the male penis is repeatedly not only the discussion topic among men, but also the subject of scientific studies. Last in 2015, British researchers evaluated 20 research projects in a large meta-study. Accordingly, the penis of European men is 9.16 cm long on average in the flaccid state. With a full erection, the average value is 13.12 cm. Average values are a median value which says nothing about the scattering of the penis size. According to the urological expert association in Switzerland and Germany, penis lengths between 7.5 and 19 centimeters are considered normal.

Your partner for the therapy of erectile dysfunction

Video: Prof. Dr. med. Diehm on the subject of erectile dysfunction

Would you like more detailed information in advance? Watch the following video. Dr. Diehm gives an overview of causes and treatment of erectile dysfunction. (German)

Do you have any questions?

At the Institute for Erectile Dysfunction you find qualified dialog partners for all questions on erectile dysfunction. If you have any questions or would like to make a consultation appointment: Call us: +41 62 824 02 42 or write an e-mail to