When we breathe we use our diaphragm in order to move air in and out of our lungs. The diaphragm is classified as a skeletal striated muscle and is capable of being voluntarily controlled (unlike other muscles such as the heart). The upwards and downwards motion of the diaphragm is one of the main factors to consider in answering the question: how many calories do you burn breathing.
Each inhalation and subsequent exhalation provides the body with oxygen and removes carbon dioxide, respectively. The oxygen is a key ingredient in allowing the release of energy via glucose. Glucose is an important type of carbohydrate and is the main source of metabolic energy in our bodies. This exchange of gases during respiration is critical to the functionality of the circulatory system, via diffusion of molecules through capillaries in the lung. These diffused gases then travel along with the blood stream.
The question how many calories do you burn just breathing is also influenced by the fact that exhalation depletes the body of water, which must then be compensated elsewhere in the body. The calories you burn breathing is taken into account in your total basal metabolic rate. This includes a variety of factors, including the motion of your heart pumping, your lungs moving and the motion of your digestive tract.
If you wish to know answers to questions such as how many calories do you burn deep breathing, then this is a matter of separating the energy consumed by your breathing from your total basal metabolic rate. Your basal rate is likely to lie between 1500 and 2000 kcal per day. The amount of this due just to your breathing is difficult to segregate from the total, but we can assume that it will be a significant percentage, as the respiratory system requires quite a high amount of energy relative to other processes in the body.