ELi5: When a baby is born, how are their lungs instantly able to breathe air if they haven’t been breathing air for 9 months?
In: Biology
Ok, will do my best for ELI5.
A few things need to happen for a fetus to transition to being a baby— they need to absorb the fluid in the lungs (helpfully called fetal lung fluid), they need to fill the lungs with air, and they need to send more blood to the lungs to bring back oxygen.
Fetuses start absorbing their lung fluid when they get closer to their due date, and then the stress of labor helps them do more. Taking a breath and filling the lungs with air helps get the last of it absorbed.
Lungs work best when they have some air in them— think of a balloon. When it’s empty the first time, you have to work pretty hard to blow air in, but once it’s stretched open a bit you can put more air in with less force. Lungs are like that too (surfactant helps with this). So crying at birth helps a baby get that first bit of air in.
Finally, lungs full of air aren’t that useful if the heart doesn’t send blood there. As the lungs inflate, it becomes easier for the heart to send blood that way (and when you take away the placenta the other way gets harder). The fetal heart has some short-cuts for blood to skip the lungs, and those close up after birth so blood goes the right way.
This works right about 90% of the time in full term babies. When babies don’t cry, we bother them so they start crying. Usually all you have to do is dry them off and rub them a bit and they will cry, but if they don’t then we will help them breathe by pushing air into their lungs. This fixes the problem most of the time and once they have air in their lungs they start acting like babies not like fetuses.
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