Erection Of The Penis

By Johnathan P Cumberwell

Your penis is normally in a flaccid (limp) state. It is flaccid because it simply wouldn’t be practical to have a constant erect penis.

But how does it become erect? And why?

It starts with a spark of sexual stimulation. She touches or kisses you, you may have a sexual fantasy, or similar.

Your brain then realizes: ‘Hi, here is an opportunity to reproduce!’ So it sends signals down your spinal cord, to your penis, that instruct your penis to fill with blood. 

Blood then starts to flow into your penis to fill it up. When your penis is filled with blood, you have an erection.

We will dig into the details of how this works in a little, but let’s first understand the anatomy of the penis.

A Brief Anatomy Of The Penis

Let’s first take a look at the main components of the penis and how they work.

There are three main erectile structures inside your penis: The corpus spongiosum and the two corpora cavernosa.

These three structures lay parallel to each other, and run from the base of your penis, all the way to the glans (the head of your penis).

The urethra, which is the tube that carries urine as well as sperm, runs inside the corpus spongiosum.

Although the corpus spongiosum does fill with blood during an erection, the overall blood pressure inside is low and as a result, it does not give much support to the erection.

The pressure inside your corpus spongiosum is low in order to allow sperm to pass through the urethra during ejaculation.

Instead, it is the two corpora cavernosa that are responsible for making your penis hard.

The two corpora cavernosa are chambers consisting of networks of open spaces, sinusoids, smooth muscle, nerves and small arteries. Together, these are called the spongy erectile tissue.

When you get an erection, these two chambers fill with so much blood that the pressure inside increases substantially.

And it is this pressure that provides the stiffness of your erection.

The tunica albuginea is another part that is essential for making a firm erection happen.

The tunica albuginea forms a layer around the corpora cavernosa. This layer is of limited flexibility. This is important, because this inflexibility enables blood pressure to build up inside the corpora cavernosa as blood flows in.

This can simplistically be compared to filling a bike tire with air:

As the inner tube (corpora cavernosa) becomes filled with air, pressure increases, the outer rubber (tunica albuginea) provides resistance and the tire becomes firm and hard.

Also, the wall between the two units of the corpora cavernosa has a number of openings that let blood flow freely between them, so that the pressure is similar in both of these chambers.

As the tunica albuginea expands outward, it also squeezes off the drainage veins at the bottom of the corpora cavernosa, which traps the blood and increases the blood pressure inside the penis.

There are three main arteries that supply blood to the penis: The cavernosal artery, the dorsal artery, and the bulbourethral artery. All three arise from an internal artery called the penile artery.

The corpus spongiosum, the two corpora cavernosa, and the tunica albuginea are surrounded by something called the buck’s fascia.

The buck’s fascia is a strong, deep, layer that sits on top of the tunica albuginea, and this is in turn surrounded by the penile skin, which is what we see externally.

Now that we have an understanding of the anatomy of the penis, let’s better comprehend how your penis becomes erect.

The Process Of A Penis Erection

For most of our history, we didn’t know (at least it wasn’t scientifically documented) how a penis becomes erect.

This changed when Leonardo Da Vinci started dissecting human bodies.

He was the first scientist to realize and document, that the penis fills with blood during an erection.

OK, so the penis fills with blood to become erect, but why is it flaccid most of the time?

Needless to say, it would probably not be practical to walk around with an erect penis at all times. That’s probably why it is normally flaccid.

But mechanically, why is the penis normally flaccid and what makes it suddenly fill with blood?

Each blood vessel consists of several layers. One of these layers is a smooth muscle. The reason your penis is normally flaccid, is because these smooth muscles inside the corpora cavernosa, contract.

When these muscles contract, they squeeze around the arteries. This forces blood out of these arteries, and therefore also out of your penis.

In other words, when these muscles contract, blood is not allowed to enter the penis (other than small amounts for maintenance purposes).

This minimal bloodflow keeps blood pressure inside the corpora cavernosa low – lower than the average blood pressure in arteries throughout the rest of the body.

This changes when you get ‘turned on’, or rather, when you experience some form of sexual stimulation. Such as a having a sexual thought, or a sensual touch.

Then dopamine gets to work.

Dopamine then sets off a chain reaction of events that eventually causes you to get an erection.

Dopamine sends signals down your spinal cord to the nerves in your penis. These signals instruct your blood vessels to produce a gas called nitric oxide.

This causes the smooth muscles in your corpora cavernosa to relax, which then causes the squeezing around the blood vessels to stop.

Now blood can flow into your penis.

As blood enters the corpora cavernosa, these chambers start to fill up. As they get filled up, pressure is increasing, blood is then pushing on the tunica albuginea, firmness and pressure continues to build, and the penis becomes erect.

This was the easy explanation of how an erection works. If you want to understand the process in more detail, you can read the next section. If not, just skip this. 😊

The Detailed Explanation Of A Penis Erection

Your penis is normally flaccid because the smooth muscles in your penis squeeze blood out.

Think of a clenched fist. When you clench your fist, you contract several muscles in your hand.

When your penis is flaccid, the smooth muscles in the corpora cavernosa and the corpus spongiosum contract, similar to a clenched fist.

And when these smooth muscles contract, blood is forced out of the penis.

Then only a small amount of blood is allowed to enter the penis for maintenance purposes.

But how exactly is the flaccid state maintained?

This is achieved when there is a high concentration of the mineral calcium in the smooth muscles.

This free calcium is what keeps the smooth muscles is in a contracted state. And unless the free calcium is reduced, the penis will remain flaccid.

OK. So how then do you get an erection?

This process starts when you experience some form of sexual stimulation. This stimulation could for instance be a kiss or a touch.

This in turn, releases dopamine in the brain. This then sets off signals that are sent from the brain via the spinal cord through special autonomic nerves.

These signals then reach the cavernous nerves that run along the prostate gland and finally the smooth muscles in your corpora cavernosa.

When the signals arrive in the smooth muscles in your penis, an enzymes called nitric oxide synthase is activated.

These enzymes sit within your endothelial cells (the inner lining of your blood vessels), and these enzymes manufacture nitric oxide.

Nitric oxide synthase creates nitric oxide from the amino acid l-arginine, and oxygen. It simply combines a nitrogen atom from l-argenine, and an oxygen atom from oxygen.

Nitric oxide synthase then diffuses this nitric oxide into the smooth muscles in the corpora cavernosa of your penis.

Nitric oxide then binds to a molecule called guanylyl cyclase.

This in turn converts a molecule called guanosine trisphosphate to something called cyclic guanosine monophosphate (‘cGMP’).

And cGMP is a key component in the erection process. Because it decreases free calcium concentrations.

cGMP also helps decrease something called the calcium sensitizing mechanism in the penis.

This means that receptors connected to proteins that help smooth muscle cells contract, decrease their sensitivity to free calcium.

This reduction of free calcium then induces relaxation of the cavernosal smooth muscle cells.

As these smooth muscles relax, blood starts to flow into the corpora cavernosa (and the corpus spongiosum), and the erection is starting to build.

The increased bloodflow expands the corpora cavernosa. This then stretches the tunica albuginea (the membrane of limited elasticity on the outside of the corpora cavernosa).

As the tunica albuginea stretches, it builds up pressure and firmness inside the corpora cavernosa.

This pressure also compresses the veins at the bottom of the penis where blood would normally flow out. This traps blood within the penis, so that your penis can remain erect.

So that’s the detailed explanation for how you get an erection. 😊

But there is one element we haven’t mentioned yet, which is also essential in the process:


Without testosterone, the steps above would not be possible.

That is because testosterone, as the primary male sex hormone, regulates, controls and supports the sexual activities that happen in you.

Therefore, in order for the erection process to work smoothly, you need enough testosterone.

Now that we understand how you get an erection, lets also look at the reverse.

How Does Your Erection End?

When you have had an orgasm, or sexual stimulation is ceased, the erection will normally fade.

The mechanism behind the end of an erection is the reverse of the process of getting an erection:

The amounts of cGMP in your penis decrease and therefore, levels of free calcium increase.

When free calcium levels increase, the smooth muscles in the penis will again start to contract.

This contraction then forces blood out of your penis, and the erection ends.

In short, both contraction and relaxation of the smooth muscles are regulated by free calcium.

Therefore free calcium regulates bloodflow in and out of the penis, and therefore also regulates how you get and lose erections.

This raises an interesting question: How do Viagra and the other pharmaceutical drugs enable you to get erections? How do they treat erectile dysfunction?

How Do Viagra, Cialis And Levitra Produce Erections?

Viagra, Cialis and Levitra and other pharmaceutical drugs are able to help many men get erections. But how do they do this?

No surprise: They target the exact same free calcium level as described above.

By manipulating your free calcium level, they can help you both get an erection, and also help you maintain the erection for longer than would otherwise be possible.

Viagra, Cialis and Levitra all do this by first acting on an enzyme called phosphodiesterase type 5 (‘PDE5’).

This enzyme helps control the level of cGMP, or rather, it breaks down cGMP.

When cGMP is broken down, the free calcium levels are allowed to rise, smooth muscles start to contract and blood will flow out of the penis.

What these drugs do is to inhibit this PDE5, so that it does not break down cGMP (or breaks it down much slower).

When there is more cGMP present, this will decrease free calcium levels. Therefore, you are likely to get an erection. Which is also allowed to remain for longer than normal.

Therefore, Viagra, Cialis and Levitra enable you to get erections by inhibiting PDE5, which increases cGMP, which in turn decrease free calcium.

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