Need-to-know anatomy

Two spongelike chambers, called the corpora cavernosa, run the length of the penis above the corpus spongiosum, which contains the urethra, the channel for urine and semen. To initiate an erection, the brain and body send sensory signals to the nerves of the penis, causing the muscles of the corpora cavernosa to relax and the blood vessels to open up. Blood flows in, and the penis expands. A membrane surrounding the corpora cavernosa (known as the tunica albuginea) traps the blood, sustaining the erection.

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Ejaculation occurs as a result of signals that travel along the sensory nerves of the penis into the spinal cord and the brain, which control the release of semen from the urethra. Ejaculation consists of two phases. In the first phase, the vas deferens (the tubes that transport and store sperm from the testes) contract to squeeze the sperm toward the base of the penis while the prostate gland and seminal vesicles release secretions to make semen. In the second phase, muscles at the base of the penis contract every 0.8 seconds and force the semen out of the penis in up to five spurts.