Let's talk about how a normal erection works with sexual stimulation. The brain sends some nerve messages to the penile arteries, the main highways that send blood to the penis. This increases blood flow. Now there's two cylinders or chambers on each side of the penile shaft. These cylinders fill up with blood until the pressure gets higher and higher in each one of those chambers. When the pressure equals the same pressure as your body's arterial pressure, this shuts down the blood exit mechanism and the chambers remain full at climax or orgasm, the whole process reverses and all the blood flows out of the penis to return it to the flacid state.