• The Respiratory System – a site made by 3rd graders from ThinkQuest

• All Living Things Need Oxygen - Learn how other animals breathe.
• Lungs : Why do we breathe? How do our lungs work? (the Kids site from the American Lung Association)
  

• KidsHealth.org
• Looking at your Lungs
• What’s a Booger? And how does it help to protect your lungs?
•  Asthma  
  

• The Heart: An Online Exploration - from the Franklin Institute.  Hear heartbeats, see an open-heart surgery (if you dare), and more
  
  
• The Circulatory System – also from ThinkQuest

• The Human Heart - Learn the different parts of the heart
  

• KidsHealth.org: All About the Heart

THE URINARY SYSTEM

• The Urinary System : Getting Rid of Waste (ThinkQuest)

• New Kidney – These ThinkQuest kids really know some stuff!

• KidsHealth.org: What Kids Need to Know about Kidneys