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While the baby is being born, everyone's eyes are focused on the star as the grand entrance is accomplished. But the spectators are waiting not just for the appearance of the baby; its first sounds are also eagerly anticipated. A loud and lusty cry is the proof that the baby has made the transition to its new environment safely and can now breathe on its own.
In order to understand the miraculous changes that take place in the first few moments after birth, it is necessary to examine the way the fetal blood circulation works. There are two critical differences between fetal circulation and newborn circulation. The first difference is that oxygen is delivered to the fetus across the placenta and into the umbilical cord. This oxygenated blood is returned to the fetus's heart and then pumped out to the rest of the body. The second difference is that the fetal lungs do not serve a function and, therefore, receive only a small fraction of the circulation as compared to the circulation requirements after birth.
These important differences are reflected in structural differences within the heart itself during the course of gestation. Although the fetal heart contains the same four chambers and blood vessels that the child or adult heart contains, the chambers and blood vessels relate to each other in different ways. For example, the two upper chambers of the heart, the left and right atria, are open to each other through a special passage (the foramen ovale) during gestation. Additionally, although a large amount of blood is pumped out to the lungs through the pulmonary artery, a lot of it is diverted to the main circulation by way of a special channel (the ductus arteriosus) before it gets to the lungs. These structural differences ensure that very little oxygen is wasted by pumping it through the fetal lungs, which are virtually useless and, in fact, are partially collapsed within the chest.
All this changes in the first few moments after birth. Probably a variety of stimuli contribute to the newborn's tremendous urge to inhale its first lungful of air. These stimuli include the rapidly falling oxygen concentration and rapidly rising carbon dioxide concentration as the umbilical vessels constrict. The compression of the fetal chest in the birth canal and its sudden release also may contribute to the urge to breathe.
When the newborn draws its first breath, its lungs expand to almost full capacity; at this point, the relationships among the organs in the chest change dramatically. The lungs, which previously could accept only a small amount of the blood leaving the heart, now can accept much more. Not only can they accept more blood flow, it is imperative that they receive it, because this is the new source of oxygen.
This dramatic shift in the amount of blood flow changes the blood pressure within each chamber of the heart. As the pressure rises in the left atrium, a flap of tissue is pushed over the foramen ovale, effectively closing this passage between the two atria. Additionally, the ductus arteriosus, the channel that previously drained blood away from the lungs, constricts, ensuring that all the blood pumped out to the lungs actually reaches the lungs. There, the blood will pick up its load of oxygen, before returning to the heart to be pumped out to the rest of the body.
The blood vessels of the umbilical cord are instantly rendered obsolete. If they have not constricted already, they will do so after the baby takes its first few breaths. This process is aided by the clamping of the umbilical cord. The umbilical vessels will atrophy and the remnant of the umbilical cord still attached to the baby will dry up and fall off. The place where the umbilical cord was attached to the baby will forever be marked by the navel, also known as the umbilicus.
These amazing changes take place within moments after birth. The baby's first cry is not a cry of protest, but a cry of life, announcing that the transition between the uterus and the outside world has been safely negotiated. For this reason, the newborn's first cry is music to the ears of obstetricians, midwives, and new parents.