All vertebrates carry out aerobic oxidative metabolism. The living cells of vertebrates must obtain oxygen if they are to carry out this metabolism, synthesize ATP, and continue to live. At the same time, the cells must be able to get rid of the carbon dioxide they produce as a by-product of their metabolism. The respiratory systems of vertebrates, whether gills or lungs, are simply mechanical devices that ensure that an adequate exchange of gas can occur: enough oxygen can be taken into the body to meet the metabolic demands of all its cells, and carbon dioxide can be removed. This chapter examines the details of vertebrate respiratory systems (i.e., the mechanics of gas exchange and the anatomy and functioning of the various types of vertebrate respiratory systems). This chapter also 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.