Erection takes place when a sensorial stimulus or a thought triggers a cascade of physiological processes in the male body. Here is the anatomical basis for these processes and the processes themselves described.

The erection process begins in the higher areas of the brain when these recognise something through the senses or deals with thoughts capable of inducing sexual excitement. Impulses are then sent down through the nervous system causing physiological processes resulting in penile erection and engorgement of the area between the legs.

The sexual activities and the erection itself will also stimulate sensorial bodies in the genital zone, and impulses are sent back from the genital area to the spinal cord and the brain. These impulses make the brain even more excited, and the brain then sends its impulses down to the genital area even more vigorously. Thus erection stimulates even more erection.

However, this stimulation reaches a peak giving the maximal feelings, the orgasm, which usually also is accompagnied with semen ejection. When this peak is reached, the stimuli for erection go away

In the penis lie three bodies consisting of a network of very elastic blood vessels having circular smooth muscles in their walls, called erectile bodies. There are two paired bodies lying at the upper side of the urethra (corpora cavernosa), and one lying around the urethra at the under-side (corpus spongiosum).

These bodies are not situated only in the visible penis, but continue under the scrotaum and between the man's legs fra backwords to the prostate region, just in front of the anus.

The muscles in the vessel walls are able to constrict the volume of the vessels, or relax allowing the vessels to widen. Vessels leading blood to or from the erectile bodies also have the ability to constrict or relax in order to restrict or facilitate blood flow.

Because the erectile bodies go so far back, the fully erct penis will be firmly held in place and not wiggle from side to side.

The erection response consists of the following steps:

- Higher areas or the brain, dealing with sensing and thoughts, recognize sensorial stimuli, or engage in thoughts that have the ability to induce sexual excitement. The stimuli can come from physical stimulation of the genital zone, from sight, smell or even hearing.

- The higher brain areas then send impulses down to the limbic system at the lower area of the brain. It is the limbic system that actually produces the feeling of excitement.

- The limbic system sends signals down the spinal cord and from here to the genital area through nerves called nervi erigentes.

- When the signal reaches the nerve ends, the chemical compound nitric oxide (NO) is released.

- Nitric oxide then spreads through the genital area and especially the penis. Nitric oxide then triggers the following reactions.

- Blood vessels leading blood to the erectile bodies relax. Then more blood flows into the penis.

- Blood vessels leading from the erectile bodies constrict, making it difficult for blood to leave the bodies.

- The smooth muscles around the vessels in the erectile bodies relax, allowing these vessels to widen.

- The blood going into the erectile bodies will then fill up in the vessels of the bodies and inflate the bodies to a much larger volume.

- The inflated bodies will get straight and hard. The growing erectile bodies will inflate the whole penis and make the penis rise.

- Since the erectile bodies continues backwards between the man’s legs, also this area swells and fixes the penis rigidly so that it does not sway from side to side when fully erected.

Sensorial bodies within the penis are triggered by the erection. Then nerve impulses are sent back from the penis to the spinal cord and the brain. This response back to the brain will make the brain even more excited. Then the brain sends even more impulses down to the genital area, making the erection response even stronger. In thus way the erection response triggers even more erection.

The arousal will often make the man stimulate himself either by sexual intercource or by masturbation. This stimulation will add greatly to the amount of impulses sent from the penis to the central nervous system and the loop back to the penis that trigger the erections.

The arousal will often lead to a sensorial climax and to orgasm. The erection will usually reach its most intense stage together with the sensorial climax.

Nightly erections and erections at wake-up time in the morning

A man's and a boy's penis get erect several times in the night. This occurs sometimes together with a dream with sexual content where the man feels sexual arousal. It occur authomatically even without sexual arousal or sexual dreams, but the feeling of being erect can cause a sexual arousal.

This authomatic process probably have the purpose of holding the penile tissue in good shape. Erections during night also occur during childhood, and at that age it is probably a process necessary for the proper development and shaping of the male genital organs.

It is common for a man to wake up in the morning with an erect penis. Often an automatic erection in the morning stimulatse sexual feelings and these feelings make the man wake up. This kind of sexual arousal early in the morning has a special vitalizing effect on the whole body.  A man can then choose to use the arousal to have sex or to masturbate, or he can choose not to do anything that brings him to orgasm. If he chooses not to have orgasm at this point, the vitalizing effect will last a long time during the day.

Does women get erection?

In a certain sense the answar is definitly yes. Women have the same structurs in their genital organs as men, that is swampy bodies filled with a mesh of elastic blood vessels. When a  women is aroused, these get filled with blood and the genital lips together with the clitoris engorge in the same way as a man's penis.