The major way that the penis gets hard isn't through X-rated Internet sites, it's through receptors on cells lining our arteries that stimulate a chain reaction that ultimately relaxes the blood vessels that go to the penis. That reaction is mediated by nitric oxide, that short-lived gas floating around in your body.
Nitric oxide is usually made by the endothelial cells that line all of our arteries. As soon as even a small amount of hardening of the walls starts with the creation of mild plaque, levels of nitric oxide plummet, so the vessel cannot dilate normally when you need a little extra blood, as when running for the bus or rummaging around in bed.
Find out more about this book:You: Staying Young: The Owner's Manual for Extending Your Warranty