Respiratory system consists of certain air channels and the lungs. The human lungs are made up of some alveoli, the total surface of which is about 100 square meters, or about fifty times that of the skin surface. Thus, each lung consists of its bronchial tree with its many air sacs and alveoli units together with other associated structures such as the blood vessels, nerves and pleura. Breathing is a mechanical process and is completed in two phases, inspiration and expiration.
In inspiration, the ribs are elevated and the diaphragm contracted and flattened, the chest cavity is enlarged. This increase in the volume of the chest cavity and lungs causes the air pressure in the lungs to fall below the atmospheric pressure, and air passes through the air passageways to the lungs to equalize the pressure.
In expiration, the ribs and diaphragm return to their original position, the chest cavity volume is decreased. The distended elastic lungs then contract and air is forced out.