Image Caption : Male Pelvis Showing Erect Penis: Medical visualization of the male pelvic region with a flaccid penis also visible are the testicles, pelvis, femurs, and surrounding musculature. There are three columns of erectile tissue in the penis: at the top (or dorsal side) of the penis are the two corpora cavernosa, and at the bottom (or ventral side) is the corpus spongiosum. Upon arousal, the arteries that supply the penis dilate and allow blood to fill the three spongy erectile tissue columns, causing it to lengthen and stiffen. The engorged erectile tissue presses against penile veins, preventing blood from flowing back out of the penis.

Penile Erection

The state of the PENIS when the erectile tissue becomes filled or swollen (tumid) with BLOOD and causes the penis to become rigid and elevated. It is a complex process involving CENTRAL NERVOUS SYSTEM; PERIPHERAL NERVOUS SYSTEMS; HORMONES; SMOOTH MUSCLES; and vascular functions.

National Center for Biotechnology Information, U.S. National Library of Medicine

How does an erection occur?

An erection occurs when blood flow increases into the penis, making it expand and become firm. Two long chambers inside the penis, called the corpora cavernosa, contain a spongy tissue that draws the blood into the chambers. The spongy tissue contains smooth muscles, fibrous tissues, blood-filled spaces, veins, and arteries. A membrane, called the tunica albuginea, encases the corpora cavernosa. The urethra, which is the tube that carries urine and semen outside of the body, runs along the underside of the corpora cavernosa in the middle of a third chamber called the corpus spongiosum.

Cross section of the penis. Labels point to the corpora cavernosa, tunica albuginea, corpus spongiosum, and urethreaCross section of the penis

An erection requires a precise sequence of events:

  • An erection begins with sensory or mental stimulation, or both. The stimulus may be physical—touch, sound, smell, sight—or a sexual image or thought.
  • When the brain senses a sexual urge, it sends impulses to local nerves in the penis that cause the muscles of the corpora cavernosa to relax. As a result, blood flows in through the arteries and fills the spaces in the corpora cavernosa like water filling a sponge.
  • 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 the erection.
  • The erection ends after climax or after the sexual arousal has passed. The muscles in the penis contract to stop the inflow of blood. The veins open and the extra blood flows out of the spaces and back into the body.

NIH/National Kidney and Urologic Diseases Information Clearinghouse


The penis, the male copulatory organ, is a cylindrical pendant organ located anterior to the scrotum and functions to transfer sperm to the vagina. The penis consists of three columns of erectile tissue that are wrapped in connective tissue and covered with skin. The two dorsal columns are the corpora cavernosa. The single, midline ventral column surrounds the urethra and is called the corpus spongiosum.

Illustration of the penis

The penis has a root, body (shaft), and glans penis. The root of the penis attaches it to the pubic arch, and the body is the visible, pendant portion. The corpus spongiosum expands at the distal end to form the glans penis. The urethra, which extends throughout the length of the corpus spongiosum, opens through the external urethral orifice at the tip of the glans penis. A loose fold of skin, called the prepuce, or foreskin, covers the glans penis.

National Cancer Institute / NIH

An erection (clinically: penile erection or penile tumescence) is a physiological phenomenon in which the penis becomes firmer, engorged and enlarged. Penile erection is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is often associated with sexual arousal or sexual attraction, although erections can also be spontaneous. The shape, angle and direction of an erection varies considerably in humans.

Physiologically, erection is triggered by the parasympathetic division of the autonomic nervous system (ANS), causing nitric oxide (a vasodilator) levels to rise in the trabecular arteries and smooth muscle of the penis. The arteries dilate causing the corpora cavernosa of the penis (and to a lesser extent the corpora spongiosum) to fill with blood; simultaneously the ischiocavernosus and bulbospongiosus muscles compress the veins of the corpora cavernosa restricting the egress and circulation of this blood. Erection subsides when parasympathetic activity reduces to baseline.

As an autonomic nervous system response, an erection may result from a variety of stimuli, including sexual stimulation and sexual arousal, and is therefore not entirely under conscious control. Erections during sleep or upon waking up are known as nocturnal penile tumescence (NPT). Absence of nocturnal erection is commonly used to distinguish between physical and psychological causes of erectile dysfunction and impotence.

A penis which is partly, but not fully, erect is sometimes known as a semi-erection (clinically: partial tumescence); a penis which is not erect is typically referred to as being flaccid, or soft.

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