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How Do Erections Work?


An erection seems like a simple thing, but it actually takes a complicated chain of events to make it happen.

Erectile dysfunction can be caused by many different physical and psychological factors, which break this important chain in different ways. That’s why there are different treatments required for ED, depending on the cause; the treatments must fix the particular link in the chain that is broken.

To understand the causes and treatment of your ED, you need to understand the chain of events that produce an erection.

The Erection Chain

This is a simplified version of the erection chain. There are many fine details that are important to doctors, but that’s more than you need to know in order to understand the basic chain.

Arousal and the Brain

Erections begin in the brain. The brain receives stimuli, which might include sights, sounds, smells, touch, or even mental images. The stimuli produce a sense of arousal, which causes the brain to produce two potent chemicals, dopamine and oxytocin.  These chemicals in turn trigger nerve impulses that are sent down the spine.

  • Arousal may be prevented by psychological conditions such as stress, performance anxiety, or relationship problems.
  • Arousal may also be prevented by low levels of the “male hormone,” testosterone.  Low levels of testosterone can cause decreased libido, or sexual desire.

Carrying the Impulse – the Nerve System

  • The nerve impulses may be blocked by conditions that damage the nervous system, including multiple sclerosis (MS), Alzheimer’s disease, Parkinson’s disease, or surgery or radiation treatments.

Circulation and Blood Flow

The nerve impulses sent from the brain produce a chemical called nitric oxide (NO)1, which in turn triggers the action of an enzyme called cyclic guanine monophosphate (cGMP). CGMP increases the size of the blood vessels that carry blood into the penis, and decreases the size of the blood vessels that carry blood away from the penis.

  • This link in the erectile chain can be broken by circulatory problems, which can prevent sufficient blood flow into the penis.  This is a very common cause of ED, especially in older men, or men in poor general health.
  • This important chemical step can also fail if your body does not produce enough nitric oxide.
  • Finally, some medical conditions may interfere with the chemical signals and reactions that are necessary to direct the blood flow into the penis.

What Happens in the Penis

The increased blood flow into the penis inflates spongy tissues, called the corpora cavernosa, in the penis, which increase in size and become “hard,” much like a balloon when it is filled with air.

  • Physical damage or trauma to these tissues can prevent an erection.

After ejaculation, the body produces adrenaline. Adrenaline is the “fight or flight” hormone; it’s release increases the blood flow to major muscles, and reduces the blood flow to smaller blood vessels, including those in the penis.  Adrenaline also increases the levels of an enzyme called type 5 phosphodiesterase (PDE5), which counteracts the effects of nitric oxide (NO).  

You can think of the effects of NO and PDE5 as balancing each other on a see-saw.  More pressure on the NO side causes an erection; more pressure on the PDE5 side softens the erection.

  • Stress can increase the levels of adrenaline and PDE5, preventing blood from inflating the corpora cavernosa.
  • If the body does not produce enough NO, or if you have circulatory problems, the chemical signals may be overwhelmed by PDE5, preventing an erection.  Popular ED medications such as Viagra and Cialis act by inhibiting PDE5, making the nitric oxide more effective.

Treating Erectile Dysfunction

Before choosing a treatment for ED, it’s important to understand the cause, and which link(s) in the erectile chain are broken. For example, oral medications like Cialis and Viagra (which inhibit PDE5, improving the blood flow into the penis) won’t help if the nerves that carry impulses from the brain are damaged.

Also remember that ED may have more than one cause.  For example, you may have circulatory problems that make it difficult (but not impossible) to have an erection.  This might cause performance anxiety, which makes it impossible for you to maintain an erection.  In this cases, treating your erectile dysfunction will mean addressing both the physical problems and the psychological issues.

For More Information

Causes of Erectile Dysfunction

Treatments for Erectile Dysfunction


  1. Don’t confuse nitric oxide (NO) with nitrous oxide (N2O) – the stuff your dentist gives you. They are entirely different chemicals. Going to the dentist will not give you an erection!

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