Here's how that works: If an artery has spent 20 years throwing parties-that is, gradually accumulating plaque and narrowing-your body has spent at least some of those years learning how to reroute blood flow around that artery. It's called collateral blood supply. It's as if a bridge you take to work every day has been closing down one lane at a time. After swearing like a character on The Sopranos, you learn-over weeks or months-to take new routes to your office. So does your body; it builds new blood vessels to help out the traffic-clogged main vessel, and that's a great adaptive response that humans have developed to survive injury.
To get technical, when vessels close, the surrounding blood-starved tissues release proteins that signal new blood supplies to develop. (Staying physically active forces tissues to demand even more blood with resulting increases in collateral blood supply; the wonders of exercise never cease.)
Now let's say you've become completely reliant on that one bridge, and one day you arrive at the tollbooth, and the entire bridge is closed down. You're stuck. Likewise, if a big clot has left its spot and has been meandering along your somewhat narrowed arteries, it could hit a spot it just can't fit through.
That can be more likely to happen with only a 50 percent clog. Why? Your blood may have flowed freely enough that your body didn't find it necessary to build extra highways and an alternate-route bridge. When that clot gets stuck in the new path, your main bridge is closed, your heart's not getting enough oxygenated blood, and the traffic jam can turn fatal. The clot has closed the artery, and the heart muscle that artery nourished begins to die. That's a heart attack.
Find out more about this book:YOU: The Owner's Manual, Updated and Expanded Edition: An Insider's Guide to the Body that Will Make You Healthier and Younger