Breathing is the result of a vacuum formed within the lungs.
The diaphragm lines the lower part of the chest cavity, sealing it off from the rest of your body. When you want to inhale, the dome-shaped diaphragm contracts, straightening itself out. This lowers the pressure in the chest cavity causing air outside the lungs to rush in to fill the space. The low pressure inside pulls in air to equalize the pressure. When the lungs expand, the cartilage at the ends of the lungs stretch, allowing room for the lungs to hold air. With the ribs expanding outward and the diaphragm lowering downward, this increases the volume of the chest cavity by about half a liter, which is the average inhaling amount.
The exact opposite is the cause for expiration. When the diaphragm relaxes, moving upwards, the chest cavity becomes less in volume, raising air pressure inside the lungs, forcing air out into the atmosphere. Muscles, such as the diaphragm, cannot push out, but only contract. When you inhale, different tissues in your chest cavity stretch. Relaxing the diaphragm allows them to return to normal size, which raises the pressure, thus forcing air out.
Respiration is the chemical process by which cells break down energy rich molecules (glucose) to get energy to perform cellular functions. Respiration is a chemical reaction which takes place inside all living cells. Cellular respiration is a complex series of chemical reactions where sugars are broken down to release energy by the use of oxygen. The key is not the gases that are released, but the energy.
Respiration involves external and internal respiration. External respiration is the exchange of gases between the alveoli and the blood. Here oxygen diffuses from the alveoli into the pulmonary capillaries and carbon dioxide diffuses from the pulmonary capillary into the alveoli.
Internal respiration is the exchange of gases between the systemic capillaries and the tissues. Here oxygen diffuses from the blood in capillaries to the tissues and this oxygen is used during respiration to 'burn' the food, releasing its energy. The waste CO2 produced diffuses from the tissues into the capillaries, and taken to the lungs, from where it is exhaled out.