Hi Tegan... before we're born we have no need for a respiratory system, as our mums do the breathing, blood circulation and oxygen etc for us. So the "water" provides a nice comfy environment where we don't bump ourselves on any sharp corners etc and our skin doesn't dry out.

But the human body needs oxygen to survive, so if we're not getting it from Mum, we have to do it ourselves (ie breathe the air)

Don't know if that helps....