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I agree that starting out with the body's systems is a good idea. My 3rd graders are currently finishing up their projects on the human body and its systems. A great online video (and other sources) you might want to start with:
http://kidshealth.org/kid/htbw/htbw_main_page.html
Your students could use the informaton from the above videos to present their particular system to the class. Or, maybe they could do a Readers' Theatre format to show how the organs work together, along with a bit of advice how to keep that system or organ healthy.
My students were divided up into system teams...researched more information and created a lovely class book,
The Important Book on the systems and various organs. The book was based on the writing frame of
The Important Book by Margaret Wise Brown.
It can be overwhelming, but I decided to keep it simple.
Have fun!
LindaR