In the Zone has some excellent resources
Introduction
Body Trivia questions
From the resources 9-11 section Adapt PowerPoint presentation - lesson 1 Use slide 3
to talk about how the heart works
They also recommend these videos
The heart and how it works
Keeping healthy - the effect of exercise on pulse rate
Heart and circulation
The try a practical activity about taking your pulse
How to take your pulse is a very good guide
Good background information here including expected pulse rates.
Habits of the Heart
The next step
understanding recovery rates
The amount of time the heart takes to return to a normal at-rest rate
after exercise is called recovery time. This is a measure of
the body's general fitness. The shorter the recovery time, the higher
the level of fitness. ( source Habits of a Healthy Heart )
from 9-11 section Powerpoint 2 slide 4
The Zone explains
'recovery rate which is measured here in beats per minute (bpm). Ask the students what they notice about the recovery rates of the people ... They should notice that the people who do the most strenuous exercise on
a regular basis (i.e. the marathon runner and the swimmer) have higher recovery
rates than those who do only light exercise (the dog walker and the boy who
cycles to school), which means that their heart rate slows down more in the
first minute after exercise.'