The best thing about this site is that it is easy for Junior High School students to understand. Most sites are too advanced for them. This site explains structure and the working of the heart and also related bits and pieces like blood vessels and the function of the blood. The pictures are great to download, unfortunately they are not animated, but the explanations of how things work are easy to understand and fun to learn about.
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Reader Reviews » Human Heart » The Heart: an Online Exploration