This program explores the need for multicellular animals to have specialized internal transport systems. These are systems that can efficiently distribute materials to and from all parts of the body, providing nutrients to cells and removing waste products. They also provide the vital link with the exchange (respiratory and excretory) systems. The evolutionary aspect of internal transport mechanisms is discussed. Following this is a detailed look at circulatory systems, including the differences between the closed systems of vertebrates and the open systems of invertebrates.