Did you know that the average adult has about 60,000 miles (95,500 kilometers) of blood vessels? Blood flows through the body in two circuits, or pathways, that begin and end at the heart. Discover more fascinating facts in Circulatory System, a title in the Body Systems series. Each title in Body Systems guides readers through the fascinating inner workings of the human body. The human body contains several complex systems that work closely together to support life and allow the body to function properly. Each book explores the characteristics and interactions of these systems, their makeup, and their importance. This is an AV2 media enhanced book. A unique book code printed on page 2 unlocks multimedia content that brings the book to life. This book comes alive with audio, video, weblinks, slideshows, activities, quizzes, and much more.