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The cardiovascular system is the first major organ system that develops and starts functioning in a growing embryo, because diffusion alone can no longer supply it with enough oxygen and nutrients. The heart forms at the end of the third week from the splanchnic mesoderm as a horseshoe-shaped primordium. Bilateral cardiac tubes fuse in the midline, and the fused cardiac tube undergoes further S-shaped bending. The heart increases in size, and several chambers (two atria and two ventricles) and the conducting system are formed.