In order to do ANYTHING with your body (or, for that matter, any living thing's body- including plants) there must be available ATP or energy. In all plants and animals the process that produces that ATP is cellular respiration.
Cellular Respiration IS NOT breathing.
Breathing is not cellular respiration. Breathing is an act that few organisms do; those that do breath are mammals, birds, and reptiles. Breathing is simply the act of getting air (which contains oxygen) into the body. Everything else has a different way of getting oxygen into the body; fish use gills, amphibians use their skin, plants use stomata. So, you may be able to "breath" just fine but if your respiration is messed up then you will die.
So, in humans (you!), cellular respiration actually starts outside our body! Here is how it goes:
A. The air has oxygen in it and we need that oxygen. . .
B. so we breath- sucking down air which contains, among other things, oxygen.
C. Our lungs (specifically our alveoli- thousands of small sacs which make up our lungs) now have a high concentration of oxygen and . . .
D. our blood, which is pressed right up against our alveloi, has a low concentration of oxygen.
E. Oxygen, therefore, diffuses, from the lungs (alveoli) into our blood and is taken to the billions of cells that make up our body.
F. The blood, now, has a high concentration of oxygen and our cells have a low concentration of oxygen.
G. The oxygen, now, diffuses from our blood into the cytoplasm of our cells.
H. The cytoplasm, now, has a high concentration of oxygen and our mitochondria has a low concentration of oxygen.
I. The oxygen, therefore, diffuses into the mitochondria where it is used during the Krebs cycle to make carbon dioxide and water.
Sugar, that has been removed from our food in the small intestine, follows the same diffusion logic as above and is ALSO brought to the cytoplasm.
However, the sugar undergoes Glycolysis in the cytoplasm which splits the sugar in two. (LE) the split sugars are called Pyruvates or Pyruvic Acid- and there are a lot of them! The Glycolysis process produces roughly 2 ATPs.
In the Transition Reaction those split sugars are brought into the mitochondria and "mashed" together to form a huge molecule. (LE) The huge molecule is called Acetyl CoA and contains a bunch of hydrogen (H) and other atoms.
As the hugh molecule enters the Krebs cycle it is broken down to form CO2 and H2O while all the other Hydrogens are taken off for the next and last stage. (LE) The Acetyl CoA is oxidized (oxygen is used to form the CO2 and H2O). The Krebs cycle is where the waste is produced in the form of CO2 and H2O which we get rid of. . . see J. Four ATPs are created.
Lastly, the Electron Transport Chain, creates one molecule of ATP for each Hydrogen that there is. (LE) During the Krebs cycle NAD bonded with the H and formed NADH which was then broken apart during the E.T.C stage and produced ATP. Ultimately, 32 ATPs are formed during this stage. It is where most of the ATP is created.
J. The mitochondria now has a higher concentration of H2O and CO2 than the cytoplasm so the H2O and CO2 diffuse out of the mitochondria.
K. Now we simply go backwards. . . water and carbond dioxide diffuse into the blood which then takes it to the lungs (alveoli) where it diffuses into the alveoli (lungs) and we exhale it out of our bodies to get rid of the waste.
Smoking retards this process because it lines the walls of the alveoli with a thick tar which impedes the diffusion of oxygen into the blood and carbon dioxide out of the blood. Obesity and high cholesterol make the blood vessels carrying the blood smaller; slowing down the rate at which the blood can get the oxygen to the cells and the carbon dioxide to the lungs!