The Respiratory Cycle

In a normal unforced breath approximately 500 ml (7-10 ml/kg) of inspired air is directed via the nares, where humidification, warming and filtration transpire, to the oropharynx and thence to the trachea via the laryngeal inlet.

From here the air traverses the main stem bronchi and is propelled to the bronchioles and terminal bronchioles. These three areas, with the trachea, constitute the conducting zone. There are 23 divisions of the bronchio-alveolar tree of which the first 16 constitute the conducting zone. From the conducting zone (CZ) the air is carried to the transitional zones (TZ) and respiratory zones (RZ) where gas exchange occurs.

Within the respiratory tree the areas where gas exchange does not occur (usually the CZ) are referred to as the anatomical dead space.

In contrast an area of the TZ and RZ receiving little or no blood is referred to as alveolar dead space. Together these make up the physiological deadspace i.e. the total volume of the respiratory tract that is not functionally involved in gaseous exchange.

Of the 500ml in the normal tidal volume, 150ml occupy the anatomic dead space so permitting 350ml to participate in gas exchange.