1. Touch, sights, sounds, erotic memories, fantasies etc, cause sexual excitement.
2. These stimuli increase signal output from a part of the brain called the Para-ventricular nucleus.
3. These signals then pass through special autonomic nerves in the spinal cord, the pelvic nerves and the cavernous nerves that run along the prostate gland to reach the corpora cavernous and the arteries that supply them with blood.
4. In response to these signals, the muscle fibers in the corpora relax, allowing blood to fill the spaces between them.
5. Muscle fibers in the arteries that supply the penis also relax, and there is an eight-fold increase in blood flow to the penis. The increased blood flow expands the corpora, then stretches the surrounding sheath (the tunica).
6. As the tunica stretches, it blocks off the veins that take blood away from the corpora cavernous. This traps blood within the penis, the pressure becomes very high and the penis becomes erect.
7. During an erection pressure in the penis is at least twice the pressure of blood in the main circulation. This is possible because the muscles of the pelvic floor contract around the base of the corpora cavernous.
8. At orgasm, the signaling from the brain changes dramatically. There is a sudden increase in noradrenalin production from nerves in the genitalia. This seems to both trigger orgasm and contract the muscle fibers in the corpora cavernous and their supplying arteries.
9. The pressure within the corpora drops, which also relaxes the tunica and so allows blood to flow out of the penis
