Friday, 7 September 2007

How many times does the heart pump in the average human lifetime?

OK, to answer this, we need a few figures.

A normal pulse rate is regarded as anything between 60 and 100 beats per minute. However, most people average nearer the 60 than the 100 mark, so we'll call the average 75.

Next, how long is the average lifespan? Well, that's obviously critically dependent on accidents of birth: being born into a poor Ivory Coast family threatens your longevity more than being born into a rich Norwegian one does. The global average is about 66 years though (the figure obviously varies slightly depending on your source), and so we'll use that as our estimate.

Then, how many minutes in a year? You may initially calculate it something like this:
60 minutes in an hour, 24 hours in a day and 365 days in a week, therefore minutes in a year = 60 x 24 x 365 = 525,600.

That's not bad, but there is actually an average of 365.25 days in a year. Why? Well, as it turns out, the earth goes around the sun every 365.24 days or so. If our calendars stubbornly stuck to 365 days, the earth would be in a different position relative to the sun on each, say, December 25th as the years flew by. The result: eventually, Christmas would fall in mid-summer for those in the northern hemisphere, and the southern hemisphere would finally get to sing "I'm dreaming of a white Christmas" without sounding silly. To prevent this, we have the leap year system, whereby every fourth year, an extra day is added to the calendar. This has the same effect as adding a quarter day on to each year, but is far more practical. Nonetheless the average is still 365.25 days, and since the 66 year lifespan figure incorporates the leap year system, it's this figure that we've got to use.

Notice that we can't use the astronomically more correct figure of 365.24 years here, since by '66 years' we mean '66 calendar years', which have exactly 365.25 days on average; to hell with reality.

So, how many minutes are there in a year? Well, 365.25 x 24 x 60 = 525960 minutes.

OK, we're finally ready to plug all the figures in:
  • 75 beats/min x 525906 min/year x 66 years = 2,603,234,700
But, since we've estimated the number of beats per minute and averaged the number of years per lifespan, I wouldn't put too much store in the accuracy of the figure. Let's round it off slightly to the more memorable average figure of 2.5 billion beats per lifespan.

In this time, it'll pump roughly 150 million litres (33 million gallons) of blood to your needy body. In a single day, the heart generates enough power to lift an average sized car a height of 15 metres.

Not bad for an organ weighing only 200g (7 oz) that uses less than 1% of the power of a lightbulb.

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