This chapter begins with splanchnic mesoderm, and explains the development of the valved, four-chambered beating heart. It discusses the changes that must take place at the foramen ovale and ductus arteriosus at birth and why they must occur. The chapter describes the circulatory arcs of the human body and the changes that occur in them at birth. The hormones carried by the circulatory system are the products of endocrine glands, which secrete their hormones directly into the bloodstream. The formation of the circulatory system also involves a good deal of morphogenesis. The chapter explores the development of blood islands and their significance and the relationship of the endocrine glands to the germ layers. It presents the relationship between hormones and the circulatory system and the role of hormones in development. In our adult lives, we possess two kinds of circulatory arcs: pulmonary and systemic.