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" What is an erection? During the process of erection, the penis enlarges and stiffens. It is a complex process that happens as a result of changes within the muscles, nerves and blood vessels of the penis. The regulation of blood flow into and out of the penis is what makes an erection possible. The anatomy of the penis   Corpora cavernosa: two cylindrical tissues run the length of either side of the penis. Like sponges, they are capable of filling with blood. When the penis is soft, the muscle fibres in the corpora are contracted. Tunica: a tough outer sheath that surrounds the corpora and limits the amount they can expand. As the tunica becomes tight, blood flowing into the penis raises the pressure within it, making it hard. Corpus spongeosum: a third cylinder of tissue between the two corpora. This contains the urethra, through which urine and semen pass out of the body. It thickens towards the tip of the penis to form the helmet-shaped glans, which is covered by foreskin in uncircumcised men. How an erection happens  Touch, sights, sounds, erotic memories, fantasies etc, cause sexual excitement. These stimuli increase signal output from a part of the brain called the para-ventricular nucleus.   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 cavernosa and the arteries that supply them with blood. In response to these signals, the muscle fibres in the corpora cavernosa relax, allowing blood to fill the spaces within them. Muscle fibres 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 sinusoidal spaces in the corpora, then stretches the surrounding sheath (the tunica). As the tunica stretches, it blocks off the veins that take blood away from the corpora cavernosa. This traps blood within the penis, the pressure becomes very high and the penis becomes erect. 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 cavernosa. At orgasm, the signalling from the brain changes dramatically. There is a sudden increase in noradrenaline production from nerves in the genitalia. This seems to both trigger orgasm and contract the muscle fibres in the corpora cavernosa and their supplying arteries. As a result of this the blood flow into the penis reduces. The pressure within the corpora drops, which also relaxes the tunica and so allows blood to flow out of the penis. This allows the penis to become flaccid again.  The inability to get or sustain an erection can happen as a result of one or more of these processes breaking down.  Fortunately, there are treatments available for erectile dysfunction. Other people also read:   Penis size and enlargement surgery: find out about the surgical and non-surgical penis enlargement treatments.  Penile prostheses: artifical supports for the penis.  Premature ejaculation: one of the most common sexual problems.  Spots on the penis: what could they be?   Based on a text by Dr Dan Rutherford, GP  Last updated 27.05.2015 "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.netdoctor.co.uk/conditions/sexual-health/a12026/the-penis-and-how-it-works/>