What is the anatomy of the penis?

The penis is made up of:

  • Two chambers called the corpora cavernosa, which run the length of the organ and contain a maze of blood vessels shaped like cavernous spaces (like a sponge).
  • The urethra, or channel for urine and sperm, which runs along the underside of the corpora cavernosa.
  • Erectile tissue, which surrounds the urethra, two main arteries and several veins and nerves.
  • The shaft, the longest part of the penis.
  • The head (glans), which is at the end of the shaft.
  • The meatus, or opening at the tip of the head where urine and semen are discharged.

How does an erection occur?

When the blood vessels of the corpora cavernosa relax and open up, blood rushes in through the cavernosus arteries to fill them. The blood then gets trapped under high pressure, creating an erection.

  • An erection begins with sensory and mental stimulation. During sexual arousal, nerve messages begin to stimulate the penis. Impulses from the brain and local nerves cause the muscles of the corpora cavernosa to relax, allowing blood to flow in and fill the open spaces. The blood creates pressure in the corpora cavernosa, making the penis expand and creating an erection.
  • The tunica albuginea (the membrane surrounding the corpora cavernosa), helps to trap the blood in the corpora cavernosa, sustaining the erection. Erection is reversed when muscles in the penis contract, stopping the inflow of blood and opening outflow channels.

How does ejaculation occur?

Sexual stimulation and friction provide the impulses that are delivered to the spinal cord and into the brain. Ejaculation is a reflex action controlled by the central nervous system. It is triggered when the sexual act reaches a critical level of excitement. It has two phases.

  • In the first phase, the vas deferens (the tubes that store and transport sperm from the testes) contract to squeeze the sperm toward the base of the penis and the prostate gland and seminal vesicles release secretions to make semen. At this stage, the ejaculation is unstoppable.
  • In the second phase, muscles at the base of penis contract every 0.8 seconds and force the semen out of the penis in up to 5 spurts.

Last reviewed by a Cleveland Clinic medical professional on 11/27/2020.


  • National Kidney & Urological Diseases Information Clearinghouse. Erectile Dysfunction: How does an erection occur? (https://www.niddk.nih.gov/-/media/Files/Urologic-Diseases/sec2-How_does_an_erection_occur.pdf) Accessed 11/30/2020.
  • Alwaal A, Breyer BN, Lue TF. Normal male sexual function: emphasis on orgasm and ejaculation. Fertil Steril. 2015;104(5):1051-1060.
  • Planned Parenthood. What's the deal with erections, ejaculation and wet dreams? (https://www.plannedparenthood.org/learn/teens/puberty/whats-deal-erections-ejaculation-and-wet-dreams) Accessed 1/11/2021.

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