The foetus gets its oxygen from the placenta and not from the lungs when in the uterus. The lungs are still growing and little blood flows through them. The foramen ovale is a flap valve that lets blood pass from the right atrium directly into the left atrium, avoiding the lungs. From the left side of the heart this blood will flow into the aorta and off around the body. The ductus arteriosus is a vessel that allows blood to flow from the pulmonary trunk through and into the aorta, again avoiding going through the lungs. At birth both of these foetal adaptations must close with the first breath and the activation of the lungs.
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