The body burns about 70 percent of its daily calorie intake when at rest or just breathing. When you breathe in, your diaphragm contracts. The muscles between your ribs work to expand your chest cavity. As your lungs expand, air travels down to your lungs, through your bronchial tubes, and into air sacs. Here the oxygen from the air passes into surrounding blood vessels, and carbon dioxide moves in the opposite direction out of your heart. Breathing out requires no effort on your part, unless you are doing physical activity. Or if you have a lung disease.
So if you think quitting smoking is work, remember that breathing is work too. And if you want to continue doing one, pretty soon you'll stop doing the other.