RESPIRA TION 22 1 



capacity was thought to be capable of affording valuable informa- 

 tion in the diagnosis of chest diseases ; but little stress is now 

 laid upon it, as it varies from so many causes. For instance, it 

 can be increased by practice with the spirometer. It is greater 

 in mountaineers than in the inhabitants of lowland plains. 



It is clear from the figures we have given that in ordinary 

 breathing only a small proportion of the air in the lungs comes 

 in direct at each inspiration from the atmosphere, and only a 

 small proportion escapes into the atmosphere at each expira- 

 tion. The greater part of the air in the lungs is simply moved 

 a little farther from the upper respiratory passages, or a little 

 nearer them ; and fresh oxygen reaches the alveoli, as carbon 

 dioxide leaves them, mainly by diffusion, aided by convection 

 currents due to inequalities of temperature, and to the churning 

 which the alternate expansion and shrinking of the lungs, and 

 the pulsations of their arteries, must produce. But that some 

 of the tidal air strikes right down to the alveoli is evident enough. 

 For the respiratory ' dead space ' that is, the capacity of the 

 upper air-passages and the bronchial tree down to the infundibula 

 is only 140 c.c., or one-third of the amount of the tidal air 

 (Zuntz, Loewy) . There is no direct way of determining whether 

 any respiratory exchange goes on through the walls of the upper 

 air-passages. But by indirect methods it has been estimated 

 that about 30 per cent, of the volume of the tidal air is pure air 

 (Haldane and Priestley). This, of course, corresponds to the 

 ' effective ' dead space. Taking the average tidal air at 460 c.c. 

 (p. 220), it is clear that the effective corresponds very closely with 

 the anatomical dead space that is to say, the respiratory 

 function of the air-passages above the point where the infundibula 

 are given off is negligible. Although such calculations can only 

 be approximately correct, the agreement is of interest. The 

 immense extent of the pulmonary surface, and the extreme thin- 

 ness of the layer of blood in the capillaries of the lungs, facilitate 

 the interchange between the gases of the blood and the gases of 

 the alveoli. 



The Amount and Variations of the Intrathoracic Pressure. 

 In the deepest expiration the lungs are never completely col- 

 lapsed ; their elastic fibres are still stretched ; and the tension of 

 these acts in the opposite direction to the external atmospheric 

 pressure, and diminishes by its amount the pressure inside the 

 thoracic cavity. In the dead body Bonders measured the value 

 of this tension, and therefore of the negative pressure of the 

 thorax, by tying a manometer into the trachea, and then causing 

 the lungs to collapse by opening the chest. It varied from 

 7*5 mm. of mercury in the expiratory position to 9 mm. in the 

 inspiratory. So far as can be judged from observations made 



