The hardware
- Corpora cavernosa
- Two parallel cylinders of spongy erectile tissue running the length of the penis. These do essentially all the work of an erection. Inside are countless small spaces (sinusoids) lined with smooth muscle.
- Tunica albuginea
- The tough, relatively inelastic fibrous sheath wrapping each corpus cavernosum. It is the reason an erection becomes rigid rather than merely swollen — and the structure that scars in Peyronie's disease.
- Corpus spongiosum
- A third, smaller cylinder on the underside carrying the urethra. It stays softer during an erection so that ejaculation is not obstructed.
- Cavernosal arteries
- The inflow. They branch into helicine arterioles that open directly into the sinusoids. Only about 1–2 mm across — which is why they show vascular disease before larger arteries do.
- Subtunical venules
- The outflow. They run just beneath the tunica albuginea, which is what makes the sealing mechanism possible.
- Ischiocavernosus and bulbospongiosus muscles
- Skeletal muscles at the base that squeeze the crura and push pressure above what blood flow alone could achieve. These are what pelvic floor training strengthens.
The wiring
Three nerve systems are involved, and they do not all push the same way.
| System | Origin | Role |
|---|---|---|
| Parasympathetic (cavernous nerves) | S2–S4 | Pro-erectile. Releases nitric oxide and starts the whole cascade. These are the nerves damaged in prostate surgery. |
| Sympathetic | T11–L2 | Anti-erectile. Releases noradrenaline, which contracts smooth muscle and ends an erection. This is the system anxiety activates. |
| Pudendal (somatic) | S2–S4 | Carries sensation from the penis and drives the pelvic floor muscles that add rigidity. |
The chemistry, in five steps
- Arousal — from the brain, from touch, or during REM sleep — fires the parasympathetic cavernous nerves.
- Nitric oxide is released from nerve endings and from the endothelium lining the blood vessels.
- Nitric oxide activates guanylate cyclase, which converts GTP into cyclic GMP.
- Cyclic GMP relaxes the smooth muscle in the arteries and the sinusoid walls. Arteries dilate, blood floods in, and the spaces expand.
- PDE5 breaks cyclic GMP down again, ending the signal. This is the enzyme that sildenafil and its relatives block.
The part most people have never heard of
Getting blood in is only half of it. The other half is the veno-occlusive mechanism, and it is elegant.
As the sinusoids fill and expand, they press outward against the tunica albuginea. The subtunical venules — the drainage — are sandwiched between the expanding tissue and that unyielding sheath, and are squeezed shut. Outflow drops to almost nothing, pressure climbs, and the erection becomes rigid. The penis seals itself.
When this step fails — because the tunica has lost elasticity, or the smooth muscle cannot relax enough to compress the veins — blood runs out as fast as it arrives. That is venous leak, and it produces the characteristic pattern: an erection that starts fine and fades within a minute or two, especially once activity begins.
Three kinds of erection, and what they tell you
| Type | Triggered by | Diagnostic value |
|---|---|---|
| Psychogenic | Thought, sight, memory, arousal in the brain | Lost early in anxiety, depression and low testosterone. |
| Reflexogenic | Direct physical stimulation, via a spinal reflex arc | Can survive spinal cord injury above the sacral level. Its presence means the local machinery works. |
| Nocturnal | REM sleep, driven centrally | The single most useful clue. Their presence means nerves, arteries and the seal are all functional — see morning erections. |
Which treatment fixes which step
This is the payoff for reading the rest of the page.
| Failing step | What that looks like | What targets it |
|---|---|---|
| Arousal signal / nerve supply | No erections at all, including on waking; after prostate surgery | Injections and vacuum devices bypass the nerves entirely |
| Nitric oxide availability | Partial erections; vascular risk factors present | PDE5 inhibitors, exercise, stopping smoking |
| Arterial inflow | Gradual decline over years; other vascular disease | Cardiovascular risk treatment; injections if tablets fail |
| Veno-occlusive seal | Starts firm, fades within a minute or two | Constriction rings, pelvic floor training |
| Sympathetic override (anxiety) | Fine alone, not with a partner | Sex therapy and CBT, often plus a short course of medication |
| Hormonal drive | Low libido as well as poor erections | Testosterone, but only if genuinely deficient |
| Structural integrity | Curvature, pain, palpable lump | See Peyronie's disease |
| All of the above, irreversibly | Nothing else has worked | Penile implant — replaces the mechanism rather than repairing it |
FAQ
Frequently asked questions
Why do I need to be aroused for Viagra to work?
It blocks the enzyme that destroys cyclic GMP, but arousal is what produces cyclic GMP in the first place. No signal means nothing to preserve.
What is venous leak?
Failure of the sealing mechanism that normally traps blood in the penis. The characteristic pattern is an erection that starts well and fades within a minute or two.
Are there muscles involved in an erection?
Yes — the ischiocavernosus and bulbospongiosus at the base of the penis raise pressure beyond what blood flow alone achieves. They are trainable, which is the basis of pelvic floor therapy.
Why does the penis go soft after ejaculation?
The sympathetic nervous system releases noradrenaline, which contracts the smooth muscle, reopens the venous outflow and reverses the whole process.
Sources
What this is based on
We link to primary sources — clinical guidelines, regulator publications and peer-reviewed research — wherever possible. Links open on third-party sites we do not control.
- [01]National Institute of Diabetes and Digestive and Kidney Diseases. Definition & Facts for Erectile Dysfunction.
- [02]Urology Care Foundation. What is Erectile Dysfunction?.
- [03]Cleveland Clinic. Erectile Dysfunction.
- [04]American Urological Association. Erectile Dysfunction: AUA Guideline (2018).