This chapter examines the circulatory system, the mechanism for transporting gases and many other substances throughout the body. Complex, three-dimensional, multicellular animals could not exist without some sort of circulatory system. The circulatory system transports needed materials such as food, oxygen, and water to cells and at the same time removes waste materials from them. In a multicellular animal, the distances that materials have to move are too great to be covered by simple diffusion. Blood travels close to every living cell in the body and helps integrate all the different parts into one functional whole. All of the other organ systems interact with the circulatory system in one way or another. Blood not only carries gases, nutrients, and wastes, it also transports hormones and distributes heat. By monitoring and regulating where the blood goes and what it contains, vertebrates can fine-tune and coordinate the efficient operation of all their organs.