How the penis forms an erection.

The most important elements involved in achieving an erection are the two chambers called corpora cavernosa. The corpora cavernosa runs the length of the penis and consists of spongy tissue comprised of smooth muscles, fibrous tissues, spaces, veins and arteries.

When a man receives sensory or mental stimulation, his brain sends a signal to his penis making the muscles in the corpora cavernosa relax. This allows blood to flow into the corpora cavernosa and flood the spaces, making the penis expand. As more blood rushes in, the pressure created makes the penis feel hard and the corpora cavernosa tubes press against the veins that carry blood out of the penis. The blood then becomes trapped in the tubes and the penis stays erect. To reverse the erection, the muscles in the penis contract, blocking the inflow of blood and opening the outflow channels.