Why do we breathe in oxygen and breath out carbon dioxide?

When oxygen is breathed in it goes to the alveoli ans in to the bloodstream where it is picked up by a protein molecule called himoglobin one molecule of himoglobin can transport
 4 oxygen molecules to anywhere in the body the himoglobin transports the oxygen to another protein
 called Cytochrome C oxidase wich makes two molecules of water for every molecule oxygen (water needs two hydrogen atoms and one oxygen) when a water molecule is made it releases energy the Cytochrome C oxidase harvests the energy then uses it for every day functions
Carbon Dioxide is released because it is in most everything we eat and is toxic to humans so we breathe it out. 

You breath oxygen in order for you to live. 

You give off carbon dioxide in order for plants to live. 

Plants and animals are in a give and take process it's also know as "mutualism". 

Plants take in carbon dioxide in order for it to give off oxygen which can be very beneficial to us. 

While in humans and animals, it's in a complete opposite where in we take in oxygen and give off carbon dioxide.