Heart rate and exercise

My son noticed his heart beating after he'd been running around a couple of weeks ago and I thought he might be interested to learn a bit more about his heart. I've got a stethoscope so we used it for this activity, you could do it by feeling for a pulse but it's harder!

I let him listen to his heart beat and mine, and we talked about what his heart does i.e. pumping blood to his lungs to collect oxygen then taking the blood with oxygen around his body to his muscles which need oxygen when they make him move (simplified but he is 3). He noticed that my heart beats more slowly than his, and he had a go at counting how many times mine beats in a minute. The counting went a bit wrong as he couldn't say the numbers quite quickly enough, but he got the idea.

Listening to his heart

We talked about how the heart rate was the number of times a heart pumps in one minute. I also explained that if we counted the number of beats in half a minute, then we could double it ('two times the number') to get the heart rate. And if we counted fo a quarter of a minute we could multiply it by 4 to get the heart rate. I'm pretty sure I lost him at some point in the logic of this, but I didn't want to try and count his heart rate over a whole minute in the next part of the activity because I didn't think he'd have the patience!

I wrote a list of things for him to do which ranged from completely resting to doing a lot of exercise. I tried to pick words that he could read himself to describe them, so the list consisted of lie, sit, stand, run and jump.  I put out some big numbers (they're magnets so they have a bit of weight and don't blow around outside) and a clock. I then got him to do the activities in order and I used the stethoscope to count his heart rate after each, counting aloud. It wasn't very scientific as he did the activity for as long as he wanted to and there was quite a lot of variability in how long afterwards he stayed still enough for me to count for 15 seconds!

Heart rates for different activities

For each heart rate measurement, he found the digits to make the heart rates I counted and put them next to the activity he'd just done.  The more energetic activities resulted in higher heart rates with a series of star jumps raising his pulse to 160 beats per minute compared to a resting rate of 92.

At the end we talked about how his heart rate was higher when he was being more active.  We also talked about why that might be - when he was exercising his muscles were making him move and using lots of oxygen so moving the blood from his lungs to his body faster helps the muscles get all the oxygen they need.  We didn't talk for long though as he found the percentage signs in the box of numbers (which also has mathematical symbols) and he wanted me to explain what they were for...