Fetal Circulation

December 14, 2007


The Book You Can't Put Down: Pregnancy

My first job in medicine involved keeping the floors of the surgery area very clean. So at the age of sixteen, I saw things most teenagers could never see. I remember in particular an operation called a C-section I saw through the small glass window that was in the center of the swinging door. I had never seen any delivery, much less a C-section delivery. But what really interested me at the time was how they were going to get that baby to breathe.

I figured that in a normal delivery a woman would go into labor when it "was time," that is, when the baby was ready to start breathing, so that when the baby popped out, of course he or she would naturally start breathing. But my teenaged grasp of cardiorespiratory physiology fell short in explaining how the baby would start breathing when "taken" via C-section.

I watched. It happened. The baby was delivered. The baby started crying and breathing. No slaps. The baby just knew to breathe. I was amazed. How did it happen?

A good novel will go through several drafts before the finished product ends up in the bookstore. In the natural course of writing, many revisions allow the author to take elements at the end of the novel, the climax or denouementand make them all the more dramatic by planting hints earlier on in the story. The more that all of the elements fall together, the more striking the release of tension at the climax of the novel.

To understand the transition between life before birth and life after birth, one has to go back to the study of the fetus. There, elements are introduced which don't really reach their climax until that moment at birth when the baby spontaneously breathes for the first time. The whole thing is a set-up, like the climax of a novel. The embryo of a developing mammal is a system of open layers of tissue that fold upon themselves, creating tubes and tubes within tubes. As the embryo grows, not all parts grow at the same rate, causing twists and kinks within these tubes. Acrobatic twists and gyrations develop that result in a bunching up of a multi-chambered convolution that will be the heart.

In the adult, oxygen-poor blood returns from parts of the body to the right side of the heart. This blood is then pumped to the lungs, oxygenated, then sent back to the left side of the heart, which then pumps it to the brain and the rest of the body where the oxygen is needed. And then this process starts all over again. And again and again, seventy or eighty times a minute, for seventy or eighty years.

But in the fetus, the lungs are collapsed. Oxygen is provided by the most miraculous organ of all, the placenta. The placenta, slapped up against the wall of the mother's uterus (womb), allows the passage of oxygen and nutrients from the mother's blood through a membrane to the baby's blood. From here, it flows through the umbilical cord into the baby. The right side/left side circulation of the born baby is not the case in the unborn baby. With the lungs collapsed, there is no need for the right side of the heart to send blood to the lungs, the blood is already oxygen-rich, thanks to the placenta. Instead, there are two short cuts that allow the blood to by-pass the lungs. One is called the ductus arteriosus and the other is the foramen ovale. The ductus arteriosus steals blood normally routed to the lungs and lets it flow straight into the aorta on to the rest of the body. The foramen ovale is actually a hole in the middle wall of the heart itself, allowing blood in the right side to flow through the wall into the left side and out, likewise, to the rest of the body. In both cases, the lungs are bypassed.

Being Born Changes All of That-Birth and Delivery

With delivery, the life line of the umbilical cord is severed. Air hunger develops, and a reflex causes the newborn child to gasp for air. Like in a good novel, the plot thickens. With this gasping, the lungs expand for the first time and convert from a crimped up, solid block of tissue to soft, air-filled bags. It's the change in the consistency of the lungs that starts all of the magic.

When the lungs are in their unborn collapsed state, it takes a lot of pressure to try to pump blood through them. When they inflate at birth, this pressure falls so that it's easy for blood to flow into them. With this sudden fall in resistance, the path to the lungs becomes less resistant than the force needed to pump blood through the ductus arteriosus and foramen ovale. The flow in the heart becomes stronger on the left, which causes a one-way flap to slam shut over the foramen ovale, closing it. The generous diameters of the pulmonary arteries to the lungs far out-measure that of the ductus. The laws of physics apply here: it is easier to flow to the lungs than through the ductus and it withers. That first gasping causes the ductus and the foramen to be bypassed, and the lungs that were previously bypassed, finally join the club.

Like a good novel, everything falls together at birth, and in the closing pages of this novel we can all breathe a sigh of relief.

With our lungs of course. We close this book and get ready for the sequel called life after birth.

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