OK here goes. Change as appropriate.
Your heart and lungs are made out of special muscles, they are special because they never get tired and keep working even when you are asleep.
Your brain has control over them, the lung muscle you can also control your self which is why you can take a deep breath when you want to.
Because heart and lungs are muscles they can be exercised to get stronger. In the case of the heart it can get too strong and big. This is what happpened with the footballer. For your heart to get that big you need to exercise really hard, every day, for about 10 - 20 years. So normal PE and playing can't make that happen.
The lady who ran the marathon was already ill with a poorly heart, she knew she might die but she still ran to raise money.
Both of them had been exercising a lot more than a boy or girl can.
If an athlete has exercised so hard that their heart muscle is big then they have a really low heart rate, it can be 40 bpm. If that is the case doctors can put a pacemaker in to make it go a bit faster or if it has stopped they can put a defibrillator in. That is what the footballer has, so if his heart stops again, the machine in his chest, which has a wire that goes into the heart will send an electric shock that will make it start again.
Now you can take your own pulse. Mummy will have a pulse between 60 - 100 bpm. You will have a faster pulse because you are smaller than mum.
You need a watch with a second hand (or a stop watch), a wrist and fingers. Put the watch where you can see it, use the fingers on one hand to find your pulse on the other wrist. Put your thumb in a straight line and the pulse will be 2-3 cm down your wrist. You need to count each pulse in 15 seconds and then multiply that number by 4. It's a lot bigger than 40 isn't it?
If mum will let you, you can balance a drawing pin upside down on your wrist and it will twitch with your pulse.
Something that happens with boys and girls sometimes is that their heart rate is irregular, this is normal for children. If your heart rate is irregular get your mum to take your pulse, while she is doing that take a big deep breath and hold it as long as you can - your heart rate will be regular for as long as you hold your breath - isn't that cool? It is because in small bodies the heart and lungs cuddle up together and son't interupt each other.
If your Dr has an ECG machine mum can ask him to use that to record your heart beat, but as long as your heart rate is higher than 40 bpm you don't need that because it is not going to stop, it is going to keep on working.