How Erections Occur

Erections cause the penis to go from a small limp organ that hangs downwards to a larger, rigid structure that stands upright to allow sexual stimulation and sexual activity/intercourse. 

The penis becomes erect when blood rushes into the erectile columns causing them to swell.  This enlargement is stopped by tough fascia tissue, inhibiting the expansion of the erectile tissues causing rigidity during an erection.  Ligaments then pull the penis into an upright position while the muscles in the root of the penis help stabilize the erection.  As the penis becomes rigid the cremaster muscles in the scrotum contract pulling the testicles close to the body to protect them during intercourse.


Erections are not necessarily under conscious control, which explains why many men can't get an erection when they want one or get an erection when they don't want to have one.  Attaining an erection is actually a reflex that is controlled by the spinal cord.  Physical and mental stimulation trigger most sexual erections while testosterone triggers nocturnal erections during sleep (although testosterone also assists with sexual erections as well).  So the only "control" men may have on attaining or maintaining an erection is based on the mental and/or physical stimulation the engage in.

Since the penis goes through transition stages between flaccid (limp) and rigidly erect  - it easiest to outline the external visual changes in the size and appearance of the penis as it becomes erect prior to diving into the details about what is going on within the various tissues of the penis.


Before going into states of erection, its important to review the flaccid states of the penis since the penis is in these states the majority of the time.

The majority of the time, the penis is flaccid.  During this state, the penis is limp and hangs down.  The arteries leading to the penis are in a moderate state of clamping down rather than in a state of relaxation (hence one reason why men get erections when they are asleep and totally relaxed).  In this stage blood enters the penis through the arteries at the same rate it leaves through the veins (see below left).

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The “compact state” is another flaccid state.  This often occurs when the flaccid penis is exposed to cold temperatures (swimming, cold weather, etc.).  When this happens, the arteries can further clamp down causing the penis to reduce in size in order to pull it closer to the body for warmth.  In this state, the penis may be very small and feel firm due to the compaction of all the erectile tissue into the much smaller space (see above right).  The penis can also become compact during physical exercise when the blood is drawn away from the reproductive system into the skeletal muscles used during exercise.


It is easiest to explain the process of how erections occur by discussing what is going on through the various stages of achieving an erection.  There are five different stages the penis tissue passes through to go from flaccid or limp to rigidly erect as seen below:


Stage 1 is the “Flaccid Phase” which we just discussed.  The penis is in this state the majority of the time.  Right before an erection the flaccid penis hangs just a little lower as shown at right.  In this stage blood is entering the penis through the arteries at the same rate it leaves through the veins. 


[ii] (Dean RC, 2005)

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Stage 2 or “Filling Phase” is the beginning stages of an erection.  The penis begins to swell and lengthen as the shaft of the penis extends downward.  There is little to no rigidity or upward movement at this stage, just enlargement. 

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Stage 3 or “Rising Phase ” is a transition phase where continued pressure and swelling allows the penis to start rising from the downward position. 

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Stage 4 or “Tumescence Phase” (tumescence means swelling or swollen) is when the penis starts becoming rigid rather than simply swollen.  At this stage the penis often is rigid enough to point in a forward direction.  The testicles also are pulled tight against the body by this stage.

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Stage 5 or “Rigid-Erect or Rigidity Phase” is the point when the penis is as large as it will get and is rigid and ready for intercourse.  At this stage the penis should be upright and hard.  For some men this may appear more like the image in Stage 4 or Stage 5+.

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Stage 5+ “Maximal Rigidity Phase” is an extension of Stage 5.  Although the size of the erect penis remains unchanged, contractions of the IC muscles pull the erection to its most upright position.  As blood pressure rises inside the erectile columns the penis may pulse or throb with each heartbeat.  This is the fullest extent and rigidity of an erection.  During sexual activity the penis often goes between stage 5 and 5+.

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There are multiple steps leading to an erection – the first step is stimulation of certain parts of the nervous system.  In fact, there are three different stimulation types that lead to erections – mental, physical, and nocturnal:

  • Mental (Psychogenic) – visual cues, thoughts, emotions, smells, anticipation, motivation, etc.  In this case the central nervous system (CNS) sends messages to erectile centers in the pelvis to start an erection.  This can also be subconscious and account for unintended erections.

  • Physical (Reflexogenic) – stimulation of the penile nerves (and in some cases the scrotal nerves).  Physical stimulation of these nerves activates the pudendal nerve and pelvic plexus which stimulates the erection centers in the spine. 

  • Testosterone (Hormonal) – areas deep in the central nervous system unrelated to psychogenic cues but linked to testosterone levels can trigger erections. Testosterone production during sleep triggers nocturnal erections during REM cycles of sleep.  Surges in testosterone may also be partially responsible for unintended erections as well (especially during puberty).

Different areas of the body are triggered with each of these types of stimulation.  Two may be occurring at once – for example, both physogenic and reflexogenic may occur when a man is both mentally and physically stimulated.  However, not all the erectile centers are in the same place in the central nervous system.  For an example, some men with spinal cord injuries may not be able to attain an erection through physical stimulation, but can attain an erection through mental stimulation. 


Once stimulation occurs, a cascade of events within the penis begin. 

  • Certain compounds/chemicals are released by nerves. 

    • Some nerves in the penis release norepinephrine which causes blood vessels to contract, thus keeping the penis from filling with blood.  Since the nerves of the penis are usually releasing norepinephrine, blood does not enter the erectile tissues and the penis remains flaccid or limp. 

    • During sexual arousal, the cavernosus nerve inside the penis releases nitric oxide (also known as NO) which directly relaxes the muscle in the arteries causing them to open to allow blood flow into the erectile tissues of the penis. 

    • Compounds are also released by endothelial cells (skin-like cells).  These cells release prostaglandins and endothelins to cause further relaxation of the erectile tissues.  This relaxation in the erectile tissues allows more blood to enter the penis through the arteries.

  • The relaxed erectile columns are then able to fill and swell with blood (Stage 2). The filling of the cavernosa cause the lengthening of the penis downward.  The filling of the spongiosum column causes the glans to swell.

  • As the corpus cavernosa swell, they trap the deep dorsal vein against the Buck’s fascia.  Since the deep dorsal vein takes blood out of the erectile columns, the erectile columns can then become engorged causing the penis to start rising upward (Stage 3).  The rigidity of the spongiosum protects the urethra from becoming pinched off by the swelling cavernosa.

  • As the columns of erectile tissue swell, they stretch the fascia enough to cause rigidity (Stage 4).  When the fascia have reached full capacity, the penis becomes fully rigid (Stage 5).  Tunica, ligaments, and muscles in the root of the penis assist an erection in standing upright at this stage and certain chemicals help the tissues to remain relaxed to maintain blood flow into the penis. 




Now let’s look more in detail at what is going on in all the structures of the penis and how this aids in an erection.

Fascia (sheets of tissue)

  • The tunica vaginalis around the cavernosa stretching to capacity is what keeps an erection from enlarging beyond a certain point – a penis can only become as large are the tunica can stretch.

  • The trebeculae surrounding the spongiosum is thinner and more flexible.  This helps keep the spongiosum flexible so semen can pass through the urethra during ejaculation.  This is why the spongiosum on the underside of the erect penis is not as rigid.  The glans is not covered by the trabeculae so this is why the glans remains completely flexible making it easier to be sexually stimulated. 

  • Buck’s facsia surrounds all three erectile columns as well as the deep dorsal vein.  As stated earlier, when the corpus cavernosa swell sufficiently, the Buck’s fascia traps the deep dorsal vein against the erectile columns shutting off blood supply and promoting an erection.

  • The Dartos fascia surrounds the Buck’s fascia.  When the dartos fascia is completely stretched this creates rigidity in an erection. The dartos fascia also allows the skin of the penis to move and slide along the shaft during sexual activity, thus increasing stimulation in the penile nerves that lead to ejaculation.

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  • The suspensory ligament attaches the base of the penis to the pubic bone behind the pubic hair (see image[i] below).  This ligament prevents an erection from being pressed into the downward position so that it can remain upright.  Men often feel the tugging of this ligament when they get an erection in their underwear and the penis is pointing downward.  The upper middle portion of the penis will begin to hurt as the erection grows – this is caused by the suspensory ligament.  Weakness or poor attachment of the suspensory ligament can cause an erection not to point in an upward direction (click HERE to learn more). 

  • The second ligament is the fundiform ligament.  This ligament attaches to the pubic bone as well, but then slings around the underside of the base of the penis.  This pulls the penis towards the body causing the erection to stand upright.  Pressing these ligaments at the base of the penis in the pubic hair region during an erection will often force the penis into a more upright position. 

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  • An increased tension in the muscles at the root of the penis creates tension thus decreasing the amount of blood escaping the erectile tissue.  The bulbospongiosum muscle clamps down on the bulb (the end of the spongiosum), thus further supporting the penis in the erect state. 

  • Contraction of the IC muscles on both sides of the root, help stabilize an erection and allow the penis to stand even further upright in the “rigid-erect” phase.[i]  The IC muscles also allow the penis to move or “flex”.  This flexion not only pulls the penis more upright but it also increases pressure strengthening an erection.  The contraction of the IC muscle can be done consciously by the man (for a few seconds only) or by reflex (especially when the glans is touched).  Friction on the glans (including against the vaginal wall during intercourse) leads to continued reflex contraction of the IC muscles.  This strengthens the rigidity of the penis thus preventing loss of rigidity during intercourse.  Image below copyrighted by Veisland Medical Images and used by permission.


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  • Once an erection is attained, it then must be maintained or loss of the erection will occur.  A chemical known as cGMP is released in response to nitric oxide which causes relaxation of the erectile tissues to help maintain the erection. 



Erections stop in a process called detumescence (Stage 6).  This occurs for different reasons. 

  • First, lack of continued stimulation leads to an enzyme called phosphodiesterase (PDE-5) breaking down cGMP that is helping to maintain the erection.  When cGMP is broken down relaxation in the veins leading away from the penis causes the erection to fail.  In some men, PDE-5 breaks down too much cGMP, thus causing him to not maintain an erection. 

  • Second, signals from a different part of the nervous system begin clamping down on the vessels reducing blood flow into the penis.  Once relaxation of penile structures cease and blood flow into the penis is decreased, the rigidity of the penis fails and the entire process that led to the erection reverses. 

  • If ejaculation has not occurred, attaining another erection is possible right away.  If ejaculation has occurred, then this stage of detumescence is called the refractory stage.  In this stage the penis remains somewhat swollen – about twice the size of the flaccid penis or something similar to stage 2 or 3 of an erection.[ii]  During this stage attaining another rigid erection or ejaculation may not possible for a period of time (in fact physical stimulation can be painful during this time).  This refractory period often becomes longer and longer as men age, but can range from 30-60 minutes in most men during their childbearing years (although three hours was seen in men in this age group).[iii]  Sometimes during adolescence and early adulthood it may be so short as to be nearly non-existent and a repeated erection and ejaculation may be possible right away. 

  • After the detumescence stage the penis resumes its flaccid state (stage 1).

[i]  Dean RC et al, Physiology of penile erection and pathophysiology of erectile function Urol Clin North Am, 2005 Nov; 32(4): 379-95.

[ii] Kanner, Bernice. (2003). Are You Normal About Sex, Love, and Relationships? p. 52. &

[iii] Bhat GS, Association Between Post-Ejaculatory Refractory Time (PERT) and Premature Ejaculation (PE)., J Sex Med. 2019 Sep;16(9):1364-1370.

Images on this page from top to bottom include:

  1. Elen Bushe/ (modified)

  2. Elen Bushe/ (modified)

  3. Elen Bushe/ (modified)

  4. Elen Bushe/ (modified)

  5. Elen Bushe/ (modified)

  6. Elen Bushe/ (modified)

  7. Elen Bushe/ (modified)

  8. Elen Bushe/ (modified)

  9. Elen Bushe/ (modified)

  10. Medicalstocks/

  11. Blamb/

  12. Elen Bushe/ (modified)

  13. Veisland Medical Images and used by permission.