320 



RESPIRATION 



Even the collapsed lungs are not wholly empty of air; indeed, a lung 

 which has once respired can never be entirely freed of its air by mechanical 

 means. The reason of this is that in their collapsed state the walls of the 

 smallest bronchioles press together and thus prevent further exit of air from 

 the alveoli. This last quantity of air is spoken of as the minimal air ( Her- 

 mann). That volume of air which can be expelled after an ordinary expira- 

 tion amounts to about 1,600 c.c. and is called 

 the reserve air. If after the usual tidal volume 

 of 500 c.c. has been inhaled, inspiration be con- 

 tinued further, one can with the greatest pos- 

 sible effort of the inspiratory muscles take in 

 some 1,600 c.c. more. This is called the com- 

 plemental air. The sum of the complemental, 

 the tidal and the reserve air (1,600 + 500 + 

 1,600 = 3,700 c.c.) represents the maximal ex- 

 tent of the exchange of air possible with a 

 single complete respiration, and is called the 

 vital capacity of the lungs. 



The vital capacity is measured by first taking 

 as deep an inspiration as possible, and then ex- 

 haling with the help of all the expiratory muscles 

 into a spirometer (Fig. 128). 



From the facts thus far discussed we reach 

 this important conclusion, that we always have 

 the power of increasing considerably the quantity 

 of inspired or expired air without exhausting the 

 capabilities of the respiratory apparatus. 



To be able to judge the effective result of 

 pulmonary ventilation, it is of great impor- 

 tance to know whether the inspired air actually 

 reaches the alveoli. The respiratory exchange 

 of gases takes place in the alveoli; but the air 

 which remains in the air passages, including 

 the smallest bronchioles, can only contribute to 

 this exchange by diffusion with the alveolar air, 

 and, in view of the small diameter of the small- 

 est bronchioles and of the frequency with which 

 air in the passages is changed, this diffusion 

 must be relatively insignificant. The only way 

 to determine whether air goes directly to the 

 alveoli is to estimate the total capacity of the respiratory passages from the 

 nasal openings to the smallest bronchioles. Knowing already the volume of 

 tidal air, we should then know whether the air passages alone were sufficient to 

 accommodate the tidal volume. Only two such direct determinations have 

 yet been made, but according to these the " noxious air space," as it has been 

 called, amounts to about 140 c.c. (Zuntz). By an indirect method, the prin- 

 ciples of which cannot be presented here, the lower limit of this capacity is 

 said to be about 100 c.c. and the upper 150 c.c. (Loewy). Of the volume 



FIG. 128. Spirometer, after 

 Hutchinson. The expired air is 

 blown into the tank B through 

 the tube E. The weight C 

 serves to offset the weight of the 

 tank. 



