The Heartbeat

What’s in a Heartbeat?

The human heart is at the hub of the circulatory system and is an exceptionally hard worker. An adult heart likely beats over 100,000 times a day. Even when you are at rest, your heart muscles work hard—twice as hard, in fact, as your leg muscles do when you sprint and when necessary, the heart can double its pace within five seconds. In adults, cardiac output varies from 5 liters [10 pints] a minute—5 liters being the approximate amount of blood in the body—to as much as 20 liters a minute during exercise.

Your heartbeat is controlled by what has rightly been called a stunningly designed nervous system. This system ensures that the heart’s upper chambers (atria) contract before its lower chambers (ventricles) by delaying the contraction of the latter by a fraction of a second. Interestingly, the lub-dup sound that doctors hear through their stethoscope emanates from closing heart valves, not from pulsating heart muscles.

As a general rule, an animal’s heart rate varies inversely with its body size—that is, the bigger the animal, the slower the heart rate. For instance, an elephant’s heart beats, on average, 25 times a minute, while that of a canary virtually buzzes at about 1,000 beats a minute! In humans the rate slows from about 130 beats a minute at birth to 70 or so in adulthood.

Most mammals appear to have a life expectancy of roughly a billion heartbeats. Therefore, a mouse, with 550 heartbeats a minute, may live close to 3 years; whereas a blue whale, with roughly 20 beats a minute, may live more than 50 years. Humans are an exception. Judged by our heart rate, our life span ought to be about 20 years. A healthy human heart, however, may beat three billion times or more and thus live in excess of 70 or 80 years!

Note: The figures are approximations. Both the heart rate and the life span of individual creatures may vary considerably from the average.