1. "Human Physiology/The Cardiovascular system", February 2012 http://en.wikibooks.org/wiki/Human_Physiology/The_cardiovascular_system

  2. "Body Systems: Circulatory System", February 2012 http://www.fi.edu/learn/heart/systems/circulation.html

  3. "Electrical Conduction System of the Heart", February 2012 http://en.wikipedia.org/wiki/Electrical_conduction_system_of_the_heart