I've always maintained that we are allotted a certain number of heart beats at birth. For this reason I've long been an advocate of only very moderate exercise. You want to be in good enough shape that your resting heart rate is nice and low but you don't want to go doing marathons so you use up your heartbeats too fast. Seemed reasonable to me but most of my friends just thought I was wacky to say it.
Turns out I was right, sort of :-) From the Cosmic Variance Blog on Discover Magazine's web site:
10. A lifespan is a billion heartbeats. Complex organisms die. Sad though it is in individual cases, it’s a necessary part of the bigger picture; life pushes out the old to make way for the new. Remarkably, there exist simple scaling laws relating animal metabolism to body mass. Larger animals live longer; but they also metabolize slower, as manifested in slower heart rates. These effects cancel out, so that animals from shrews to blue whales have lifespans with just about equal number of heartbeats — about one and a half billion, if you simply must be precise. In that very real sense, all animal species experience “the same amount of time.” At least, until we master #9 and become immortal. (Amazing talk by Geoffrey West.)
It's all about time, very nice article so read the whole thing here.