The document summarizes the development of the heart and fetal circulation. It begins by discussing how the heart develops from two endothelial heart tubes that fuse to form a single tube. It then details the formation of the atria, ventricles, and septa. The document also describes the fetal circulation, where oxygenated blood travels from the placenta to the fetus through the umbilical vein and deoxygenated blood returns through the umbilical arteries. After birth, the ductus arteriosus, foramen ovale, and ductus venosus close, changing the circulation to receive oxygenated blood from the lungs rather than the placenta.