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 (corpus spongiosum).

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. The erectile chambers are not only confined to the penis, but continue in the area between the legs all the way backwards to the anal area.

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.