Rather to my surprise this seems to be one of the more difficult topics to understand. In order to try to clarify things we talked about the heart being a pump and what a pump does-moves gas or liquid from one place to another. We went to have a look at our central heating pump and talked about pumps that put air into bicycle tyres, petrol into cars and air into fish tanks.
The book suggests a model made using square biscuits and coloured icing. The blue icing represents deoxygenated blood and the red oxygenated blood. The atria are at the top of the picture and the ventricles at the bottom. This is a very schematic picture as the vena cavae are shown at the top as one vessel and the pulmonary artery and aorta are at the bottom of the picture.
We added some of our own models: a playdough model of the heart, with blue for the right side of the heart and red for the left.
This time, we were able to make the left ventricle look larger than the right.
Of course, we added to this by colouring and labeling a heart diagram.
In order to talk about how much blood goes round the body, we measured out blood volumes. First for an adult and then for a six year old. A six year old has, according to the calculation we used of 80ml per kilogram, a blood volume of about 2 litres (about 4 pints). To make this realistic, we added some red food colouring. It looks a surprisingly small amount sitting in a washing up bowl!
The Mayo clinic have a particularly useful simple video of the heart which also helps explain its function.
Do let me know about other ideas for explaining about heart function to younger children.