PBS
Pbs Learning Media: Amazing Heart Facts
This feature from the NOVA: "Cut to the Heart" highlights facts about the heart, including its size and placement, and will help you to understand the importance of this wondrous organ in our bodies.
University of Illinois
University of Illinois Urbana Champaign: The Heart of the Matter
A good quick look at the comparative anatomy of the heart of a typical bird and the human heart. Gives some of the facts behind the differences.
National Cancer Institute at the National Institutes of Health
Seer Training Modules: Introduction to the Cardiovascular System
Self-guided learning activity where students learn about the structure and function of the human cardiovascular system. There is a short quiz at the end of the lesson to check for understanding.
Other
Dr. Saul: Biology in Motion
This interactive site provides animations and activities in several areas of physiology. The images are simple, but clear. The accompanying text provides a good description of the process being shown.