The ultimate importance of the pulmonary ventilatory system is to continually renew the air in the gas exchange areas of the lungs, where the air is in proximity to pulmonary blood. These areas include the alveoli, alveolar sacs, alveolar ducts and respiratory bronchioles. However, during normal quiet respiration, the volume of air is only enough to fill the respiratory passageways down as far as the terminal bronchioles, with only a small portion of the inspired air actually flowing all the way to the alveoli. Some of the molecules in the inspired air travel the final short distance to the alveoli through diffusion, but most never reach the gas exchange areas of the lungs, instead going to fill respiratory passages where gas exchange does not occur, such as the nose, pharynx and trachea. This air is called dead space air because it is not useful for the gas exchange process; the respiratory passages where no gas exchange takes place are called dead space. On expiration, the air in the dead space is expired first, before any of the air from the alveoli reaches the atmosphere. Therefore, the dead space does not aid in removal of the expiratory gases from the lungs.