In the fetus, only a little blood passes through the lungs because the task of adding oxygen to the blood and filtering out waste gases is done by the mother’s placenta. Before birth, most blood bypasses the lungs by flowing through two openings in the heart. With a baby’s first breath the lungs expand, triggering changes so that all blood flows through the lungs to be oxygenated.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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