How Birds Get Oxygen Inside Their Eggs

Look inside this incubator. These eggs were laid 21 days ago and this one is just about to hatch. If you listen you can hear the chick pecking at the inside of its shell. Soon it will break through and take its first breath of fresh air.

It's a dramatic moment, that first breath - one shared by so many creatures - including ... Us. But hold on, think about this:
When you were in the womb you got oxygen from  your mother
through your umbilical cord. But for the last 21 days this chick has been
cut off from its mother - sealed inside an egg.

So how does it get oxygen? An egg seems like a perfectly self-contained system. The yolk and the white contain all the nutrients you need to build a baby chick. As with a human baby,
all this construction requires oxygen and that's the one thing that isn't stored
inside the egg. So where does it come from? Well take a look at this.

When you magnify an egg's shell a thousand times you can see the calcium carbonate crystals
that make up the shell and here and there -- tiny holes. One thousandth of an inch across. And these tiny holes let outside air filter in. So oxygen can pass through the shell but the
chick growing inside doesn't have working lungs yet.

How does it get that oxygen into its bloodstream? Well, a few days after an egg is laid something amazing happens. When you hold a fertilized egg up in front
of a bright light, you can see it: a delicate network of blood vessels that grows
out of the embryo's abdomen and presses up against a membrane
just inside the shell. Oxygen from the air comes in through the tiny
holes in the shell then diffuses into the embryo's blood. And the growing chick gets rid of carbon dioxide
at the same time.

It all looks remarkably similar to your early
days in the womb There was a yolk sac, at least at first, and a network of blood vessels growing out from the place where your belly button now is. But instead of pressing up against the edge of a shell your blood vessels reached the wall of the womb where they joined with an outer membrane to
form the placenta. In the placenta, oxygen from your mother diffused
into your bloodstream. It really is an exact mirror of what's going
on in the eggs of birds and reptiles.

While all this is happening, lungs are developing. We humans don't fill those lungs with air
until after we're born. But chicks get a head start. That's because the whole time oxygen is coming
in through the shell.

Moisture is slowly evaporating out. That creates an empty space that gradually
fills with air. A day or so before the chick is ready to hatch, it starts to move. It punctures that air pocket and fills its lungs.

It then has just enough oxygen to battle out of the egg and take its first breath of fresh air. This is Skunk Bear, NPR's science show Please subscribe to our channel and check out our other videos!.