How Does a Male Erection Occur?

The penis contains two chambers called the corpora cavernosa and the corpous sponglosum, which run the length of the organ (see figure 1). The corpora cavernosa is surrounded by a membrane, called the tunica albuginea. The spongy tissue within contains smooth muscles, fibrous tissues, veins, and arteries. The urethra, which is the channel for urine and ejaculate, runs along the underside of the corpora cavernosa and is surrounded by the corpus spongiosum.

A male erection begins with sensory stimulation, mental stimulation, or both. Impulses from the brain and local nerves cause the muscles of the corpora cavernosa to relax, allowing blood to flow in and fill the spaces. The blood creates pressure in the corpora cavernosa, making the penis expand. The tunica albuginea helps trap the blood in the corpora cavernosa, thereby sustaining erection. When muscles in the penis contract to stop the inflow of blood and open outflow channels, erection is reversed.