The blood vessels begin forming when the embryo can no longer be adequately nourished by simple diffusion between cells. The development of internal organs requires that nutrients be carried directly to the cells involved. In contrast, the heart has a much more complicated origin. It is formed from the embryo's outer cells and remains outside the embryo at first. Later, it is incorporated into the embryo and reaches its final position in the thoracic region. At first, the heart is rectangular (an evolutionary vestigial form). Later, it curves in, rotates and acquires its familiar form.
Fetal lungs are not functional, as oxygen is received from the umbilical veins coming from the mother, while carbon dioxide is eliminated through the umbilical arteries. Thus, a major part of the circulation is "shunted" through the foramen ovale, an opening between the two sides of the heart and through the ductus arteriosus, a canal linking the aorta (the main artery leaving the left side of the heart) to the pulmonary trunk (which exits from the right side of the heart). The chambers of the heart are not separate until the end of gestation and the first hours after birth. In contrast, the ductus arteriosus does not close until after birth, once the lungs have started functioning.
This developmental phenomenon can lead to anomalies such as a defect in the septum of the heart, persistence of the ductus arteriosus, or an improperly-placed heart.
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