What goes on inside your penis when you get an erection?

Good blood flow to the penis is vital to get an erection suitable for satisfying sex.

There are two main blood vessels that bring blood into the penis so you can get an erection. These are called the deep central arteries.

The first picture below shows how the inside of the penis looks when the penis is soft. If you look at this you'll see that the deep central arteries run through the middle of a spongy substance called erectile tissue. The blood vessels deliver the blood to the erectile tissue and, as this tissue fills with blood, it becomes firm and swells, and this gives you an erection.
How is an erection maintained?
To keep an erection, your body stops the extra blood that has been sent to the penis from leaving.

The second picture below shows how the inside of the penis looks when the penis is erect. If you look at this you'll see that your body keeps the blood inside the swollen erectile tissue by shutting off the blood's exit points (or veins). By doing this, the blood is trapped inside your penis. This allows your penis to stay erect.