Study illustrations of the heart. The heart contains valves which direct the flow of blood in one direction. Blood is forced through the flexible membranes which form the valves. Once the blood passes through the membranes, the valves collapse into a barrier, preventing the blood from flowing backwards. Students can find examples of pumps and valves in facets, aerosol sprays, automobile fuel pumps, and many household items in daily use.
Some beverage containers use a valve that resembles the valves in the heart.
Point out to the students that the heart is a muscle and not a mechanical pump. It can also be pointed out that comparisons of this kind are known as reasoning by analogy and are an important part of scientific reasoning. Hear some different heart rates and try to guess which is which. Site done by the Franklin Institute's online heart exploration. Along with a basic diagram of the heart, this page by Dr. Carl Bianco, M.D. gives an overview of how the heart functions, especially concerning the heart's chambers and valves.