– A Miracle at Birth –
How does a fetus breathe? Do its lungs work? How does it get oxygen? Are the kidneys functioning? Where does the urine go?
In a normal human, by breathing, the lungs receive oxygen. The blood from the right side of the heart goes into the lungs where it picks up the oxygen. This oxygenated blood is sent back to the left side of the heart for distribution to the body. The kidneys filter out liquid wastes and send them to the bladder. By urination, this waste water is eliminated from the body. How does the fetus do these functions?
The fetus in utero is a marvel of isolation. All bodily functions take place while confined inside the body of the mother by special adaptations of the various organ systems.
Since there is no air in the womb, the fetus cannot “breathe” air. The fetus does make small “breathing” motions with some minimal motion of the liquid that fills the lungs. The presence of the lung liquid creates a high pressure in the lung arteries, effectively stopping most of the blood flow from the right side of the heart.
Instead, the oxygen comes to the fetus from the placenta through a blood vessel (umbilical vein) in the umbilical cord. This oxygenated blood flows partly to the fetal liver and shunts partly through a special vein (ductus venous) into the large vein (inferior vena cava), going directly to the fetal heart.
In the fetal heart (since this blood already has oxygen) 95% of this blood bypasses the lungs through a hole in the heart (foramen ovale), going from the right side to the left side of the heart and out to the body. What little blood that tries to flow to the lungs (through the pulmonary artery) is shunted through a special short artery (ductus arteriosus) into the large artery (aorta) going to the body.
The urine from the bladder flows through a special tube (urachus) from the top of the bladder to the umbilical cord into a special sac that is part of the placenta.
At birth – the miracle happens, the conversion from life in the uterus to life outside.
With the first breaths, the lungs expand, bringing in oxygen. The decreased pressure in the expanded lungs allows the blood to flow from the right side of the heart into the lungs. The increased oxygen coming to the heart constricts the special small artery (ductus arteriosus), stopping the shunt of blood away from the lungs. And the increased blood flow from the lungs to the heart closes the flap (septum primum) over the hole in the heart (foramen ovale), stopping the shunt of blood away from the lungs.
Within the first few seconds of life, the fetal circulation is converted to newborn circulation. The blood vessels of the umbilical cord also constrict, shutting off blood flow to and from the placenta. And the urachus constricts, shutting off the flow of urine to the placenta.
All of these special vessels remain as small fibrous cords, reminders of the miracle of life which occurs at birth.