A pulse at the wrist

Lay two fingers, not the thumb, which has a pulse of its own, gently on the inside of your wrist, just below the thumb. The small tap you feel is your heart pushing blood along.

Two fingers on the wrist, below the thumb. The small tap is the heart pushing blood.

Wrist: pulse $c$ taps in $15$ s.

Count the taps for fifteen seconds. Suppose you count eighteen. A minute holds four lots of fifteen seconds, so multiply by four: seventy-two beats a minute. This is only an example. Yours will differ, and it changes through the day.

Count for $15$ s: $c=18$. A minute holds $4$ lots of $15$ s, so the beats per minute are $p=4\times 18=72$ (only an example).

$c=18$, so $p=4c=72$.

Now run on the spot for a minute and count again: say twenty-six taps in fifteen seconds, which is a hundred and four a minute. When you work harder, your heart does too.

After a minute of running: $c=26$, so $p=4\times 26=104$. More work, more beats.

$c=26$, so $p=4c=104>72$.

Here is the surprise. At seventy-two a minute, your heart beats a hundred and three thousand, six hundred and eighty times in a day, and you never counted a single one.

In a day: $72\times 60\times 24=103680$ beats, and you counted none of them.

$72\times 60\times 24=103680$.