When you breathe in, oxygen is being transported into your lungs. But how does the air we breathe, help our body function? The oxygen we brethe in goes through our pharnyx all the way down threw the bronchi. The air passes through the pharynx(part of the system that prevents food from entering the lungs) it is transported into a serise of tubes and stored in the alveolus.
From there the heart sends the red blood cells over that don't have any oxygen but CO2 instead, from there the alveolus swicth the oxygen for the CO2 in the cells. then the ciculatory system takes over and sends the oxygen rich cells threw the body. When we exhale, the carbon dixode we do not need is let out into the air. At this point our lungs APEAR to become smaller but it's actually the diaphragm is compressing the lungs squzzing out the CO2, kind of like sitting on a Whoopie cushion we squeze the CO2 out. The diaphragm my perform this acttion but what we se on the out side is diffrent because what we see are diffrent musles responding to the diaphragm's motion.