Composed of the heart, blood vessels, and blood, the circulatory system delivers oxygen and nutrients to every tissue in the body. At the center of this incredibly complex system is the heart, a strong muscle that continuously pumps blood throughout the body. Striving to promote a basic understanding of the fundamental physical and biological principles underlying circulatory functions, ""The Circulatory System"" describes the anatomical features of the system and examines how it responds to a broad range of challenges, such as increased activity, the microgravity of space, and hemorrhage.