system
Given the following post, respond as if you're commenting on Reddit:
human
How or why do babies know how to start breathing once they are born, yet they can live inside a fetus and breath no liquid in?

Does this mean, if some how we could oxygenate our blood and rid our selves of carbon dioxide via a machine. we could stop breathing?
gpt
When babies are inside the womb, oxygen is provided via maternal circulation.  The mother breaths in and oxygenates her blood, and some of that blood is shunted towards the placenta, where gas exchange can occur with the fetal circulation.  Fetuses have a different type of hemoglobin (the oxygen carrying molecule in red blood cells), that bind oxygen tighter than adult hemoglobin, which allows this exchange of gases from one circulation to the other.  The fetal circulation avoids circulating blood to the lungs by shunting blood from the right side of heart (pulmonary circulation) to the left side of the heart (systemic circulation)

When babies are born, there is a tremendous amount of stress, and babies will often cry.  This influx of air changes the pressure inside the lungs, and it will inflate the lungs.  Also, over time, those shunting mechanisms will close or atrophy, allowing normal right sided-->pulmonary, and left sided-->systemic circulation.  This all occurs gradually over a matter of days to weeks as the baby will begin relying on its own circulation.  

Yes, technically, if we can oxygenate ourselves via circulation and rid CO2 some other way, we can not breath through our lungs.  But this isn't very practical.  Having an IV access would be open access from the circulation to the outside world (increased risk for infection), and having a machine to remove our CO2 sounds a little silly, when our body is doing a damn good job!  

Patients who often cannot breath on their own either because of disease or trauma are often mechanically ventilated, but it usually just assisting the ventilation-perfusion mechanism that we already have.