How do chicks breathe inside their shells?

Eggshells have natural ventilation systems to keep the inhabitant healthy

The hard shell of an egg is there to serve a purpose, to protect little chicks floating inside until they are ready to hatch, surrounded by protective egg white (called albumen), which also provides protein. The embryo develops within the yolk and uses the yolk and albumen to feed. Unborn chicks tucked up safely in their eggs have an abundance of nutrients – but what about oxygen? How do chickens (or even reptiles) access air when they are in a seemingly airtight container? It seems the egg is a perfect place for a developing chick, aside from the lack of fresh air.

Mammals get their nutrients and oxygen through the umbilical cord but bird chicks are able to absorb oxygen through the wall of the shell. An egg’s surface cleverly allows gas in and out while still protecting the fragile contents.

They may not have umbilical chords, but chicks have a special way of accessing oxygen.


But how?

The egg needs to be in contact with air for the chick to survive. If you were to place a fertilised egg underwater, the chick would eventually be starved of oxygen and drown. The embryo draws in oxygen from the outside air with the help of a sac called the allantois. This is attached to the chick’s gut at one end and the inside surface of the shell at the other end. Oxygen then diffuses through the shell and through the blood vessel of the sac. Carbon dioxide leaves the egg in the same way. The waste flows through the allantois sac and is expelled into the air. This is essential to the chicks health as keeping unwanted waste inside the egg and allowing it to build up over time probably wouldn’t be good news for the chick.

Air doesn’t magically pass through the shell, but rather through tiny pores, or holes in the shell wall. Each eggshell is made of calcium carbonate and known as a semipermeable membrane, with a network of up to 17,000 small pores for air and water to pass through



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