236 RESPIRA TION 



quantity the name of supplemental or reserve air has been given. 

 After the deepest expiration there always remains 1,000 to 1,200 c.c. 

 of air in the lungs (Durig), and this is called the residual air. After 

 a normal expiration following a normal inspiration the lungs still 

 contain stationary air to the amount of about 2,500 c.c. 



The term vital or respiratory capacity is applied to the quantity 

 of air which can be expelled by the deepest expiration following the 

 deepest inspiration, and amounts in an adult of average height to 

 3,500 or 4,000 c.c. The maximum quantity of air which the lungs 

 can contain is evidently equal to vital capacity plus residual air. 

 At one time the vital capacity was thought to be capable of affording 

 valuable information 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. 



The Dead Space. 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 expiration. 

 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 inequali- 

 ties of temperature, and to the churning which the alternate expan- 

 sion 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. 235), 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 b2 

 approximately correct, the agreement is of interest. Some observers 

 have stated that great variations occur in the size of the dead space 

 with changes in the depth of respiration, its volume being increased 

 four or even eight times by very deep breathing. This, however, 

 is incorrect, although with maximum expansion of the lungs the 

 increase may amount to 100 c.c. (Krogh and Lindhard, Pearce). 



The Amount and Variations of the Intrathoracic Pressure. In 

 the deepest expiration the lungs are never completely collapsed ; their 



