Mechanism of respiration

Air containing oxygen enters the body through the nose and mouth. From there it passes through the pharix or throat on its way to the trachea (windpipe). The trachea divides into two main airways called bronchi upon reaching the lungs; one bronchus serves the right lung and the other the left. The bronchi subdivide several times into smaller bronchi, which then divide into smaller and smaller branches called bronchioles. These bronchi and bronchioles are called the bronchial tree because the subdividing that occurs is similar to the branching of an inverted tree. After a total of about 23 division, the bronchioles end at alveolar ducts. At the end of each alveolar duct, are clusters of alveoli (air sacs). The oxygen transported through the respiratory system is finally transferred to the bloodstream at the alveoli,

The trachea, main bronchii, and approximately the first dozen divisions of smaller bronchi have either rings or patches of cartilage in their walls that keeps them from collapsing or blocking the flow of air. The remaining bronchioles and the alveoli do not have cartilage and are very elastic. This allows them to respond to pressure changes as the lungs expand and contract.
Blood vessels from the pulmonary arterial system accompany the bronchi and bronchioles. The blood vessels also branch into smaller and smaller unit ending with capillaries, which are in direct contact  with each alveolus. Gas exchange occurs  through this alveolar-capillary membrane as oxygen moves into and carbon dioxide moves out of the bloodstream. Although the 300 million alveoli found in the lungs are microscopic, they have a total surface area equivalent to the size of a tennis court.
Diffusing capacity measures the ease with  which gas exchange takes place between the alveoli and capillaries. Certain lung diseases affecting the alveoli and capillary walls can interfere with diffusion and reduce the amount of oxygen  reaching the bloodstream.

The movement of air into and out of the lungs is called ventilation. The contraction of the inspiratory muscles (principal inspiratory muscle is the diaphragm) causes the chest cavity to expand, creating a negative pressure. The resulting flow of air into the lungs is called inspiration. During the maximal inspiration, the diaphragn contracts forcing the abdominal contents downwards and outwards. The external intercoastal muscles, found between the ribs, are also involved.  The muscles contract and raise the ribs during inspiration, thus increasing the diameter of the chest cavity. In addition to these muscles, the scalene muscle and the sternomastoid muscle in the neck may be employed during extreme ventilation or in condition of respiratory distress.

Normal expiration is passive process resulting from the natural recoil or elasticity of the expanded lung and chest wall (however, when breathing is rapid, the internal intercoastal muscles and abdominal muscles contract to help force air out of the lungs more fully and quickly). A lung can be viewed as the opposite of a sponge. When a sponge squeezed and released, its elasticity causes it to rebound to its larger initial size. At the end of an inspiration, the elasticity of the lung causes it to return to its smaller inter-breath size. The ability of the lung to do this called elastic recoil.

The degree of stiffness  or compliance of the lung tissue affects the amount of pressure needed to increase or decrease the volume of the lung. Lung compliance can affect elastic recoil. With increasing stiffness, the lung becomes less able to return to its normal size during expiration.

The amount of air flow resistance can also affect lung volumes. Resistance is the degree of ease in which air can pass through the airways. It is determined by the number, length and diameter of the airways. An individual with a high degree of resistance may not be able to exhale fully, thus some air becomes trapped in the lungs.