A great tutorial on the heart!
https://www.khanacademy.org/science/health-and-medicine/circulatory-system/fetal-circulation/v/flow-through-the-heart
Read morehttps://www.khanacademy.org/science/health-and-medicine/circulatory-system/fetal-circulation/v/flow-through-the-heart
Read morehttp://earthsky.org/?p=213229
Read morehttp://scienceline.ucsb.edu/getkey.php?key=1076
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