The purpose of the breathing rhythm is to ventilate the lungs to allow delivery of oxygen to the alveoli, and elimination of the waste gas carbon dioxide from the alveoli As a consequence of gas exchange at the alveoli, there are differences between the composition of inhaled and exhaled air Another factor that contributes to the differences found between inspired and expired air, is the dead space content The dead space is the region of the respiratory tract where NO gas exchange takes place Gas exchange only takes place across the thin walls of the alveoli trachea bronchi bronchioles The air filling the trachea, bronchi and bronchioles is unavailable for gas exchange and is said to occupy dead space A healthy adult, at rest, inspires approximately 600 cm 3 of air of which about 150 cm 3 fills the airways The volume of air actually reaching the alveoli is thus about 450 cm 3 As the air passages are never completely emptied of air, there is only a partial replacement of air in the lungs Composition of Inspired and Expired Air

The table below can be used to explain what happens to air as it enters and leaves the respiratory system It is important to realise that the lungs can never be completely emptied of air; even following a forced expiration, air remains within the alveoli and this amount of air is called the residual volume Composition of Inspired and Expired Air The Relative Composition (% by Volume) of Inspired, Expired & Alveolar Air Gas Inspired air % Expired air % Alveolar air % Oxygen Carbon dioxide Water vapour Nitrogen

The volumes of air inspired and expired in different circumstances, can be measured using an instrument called a spirometer The spirometer consists of a large tank of water, onto which rests a large, and very light perspex lid A nose clip is placed on the subject to prevent any air being lost from the system through the nose tank of water light, perspex lid A counterweight on the edge of the lid is used to balance the box, so that its edges just rest under the surface of the water counter -weight A series of pipes lead from from the air under the lid of the box to the mouthpiece mouthpiece A set of valves ensures that inspired and expired air travel along different pipes valves The subject breathes air into and out of the space under the lid via the mouthpiece Expired air is passed over soda lime to absorb CO 2 gas thus preventing the subject from inspiring increasing amounts of this gas Volume changes associated with breathing are recorded with a pen from the lid onto a rotating drum (kymograph) kymograph soda lime Measuring Lung Volumes

time The TOTAL LUNG CAPACITY is therefore the sum of of the vital capacity and the residual volume tidal volume inspiratory capacity expiratory capacity expiratory reserve volume inspiratory reserve volume vital capacity residual volume Spirometer tracings can be used to determine a variety of physiological measurements such as metabolic rate, breathing rate and oxygen consumption total lung capacity