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Erection all begins with arousal (except while you sleep). Sexual stimulation triggers the release of nitric oxide, a chemical that causes smooth muscles lining blood vessels to relax. This makes the tiny blood vessels inside the penis's 2 chambers to fill with blood. At the same time, muscles tighten around veins to prevent blood from leaving. When more comes in and less goes out, the penis becomes engorged with blood causing it to get longer, wider and stiffer.
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