58 VERTEBRATE RESPIRATION 



the pleural membranes and because of its elasticity and tendency 

 to draw away from the thoracic wall the pressure within the 

 pleural cavity (intra-thoracic pressure) is usually 3 to 4 mm. Hg. 

 less than atmospheric (fig. 18b). At rest the pressure within the 

 lung {intra-pulmonic) is atmospheric but during inspiration the 

 fall in intra-thoracic pressure results in the intra-pulmonic 

 pressure becoming less than that of the air and consequently air 

 flows into the lung and increases its volume. The pressure within 

 the lung therefore returns to atmospheric and increases still 

 further at the beginning of expiration when the volume of the 

 pleural cavity decreases and the consequent increase in pressure 

 is transmitted to the lungs and reduces their volume. 



The total capacity of the lungs in man is about 5| litres. Even 

 following the most forced expiration \\ litres or more of this air 

 cannot be forced out of the lungs. The amount which can be 

 exchanged as a result of forced inspiration and expiration is 

 called the vital capacity. Normally this lies between 3 and 4 litres 

 but may be 5 to 6 litres in an athlete. The volume of air inspired 

 and expired during normal breathing is the tidal volume. An 

 average figure is 500 to 600 ccs. Only part of this air reaches the 

 alveoli, however, as the rest fills the air tubes etc., which lead to 

 the respiratory epithelium. The air contained in these tubes 

 forms the so-called dead space which is about 140 ccs and hence 

 the volume of air actually reaching the alveoli at each breath 

 {the alveolar air) is little more than 360 ccs. This volume of 

 air which enters the alveolar spaces becomes mixed with 

 some 2J litres of air that is already present. Consequently, 

 the effect of such a relatively small volume on the composition 

 of the total alveolar air is only slight, probably less than J %. 

 For example, the CO 2 content of the alveolar air at the end 

 of expiration is about 5-70%. At the end of the next inspira- 

 tion it is 5-54%, the slight dilution being produced by the 

 intake of 360 ccs of air. It is clear therefore that the alveolar 

 air may remain relatively constant because of its buff'er posi- 

 tion between the atmospheric air and the tension of gases in 

 the blood. The whole of the regulatory mechanisms governing 

 respiration are directed to the maintenance of the composition 



