RESPIRATION 411 



After the expulsion of the reserve volume there yet remains in the 

 lungs an unknown volume of air which serves the mechanic function of 

 distending the air-cells and alveolar passages, thus maintaining the 

 conditions essential to the free movement of blood through the capil- 

 laries and to the exchanges of gases between the blood and alveolar air. 

 As this volume of air cannot be displaced by volitional effort, but 

 resides permanently in the alveoli and bronchial tubes though constantly 

 undergoing renewal, it was .termed 



41 The residual volume, the amount of which is difficult of determination, 

 but has been estimated by different observers at from 800 to 1600 c.c. 

 with an average value of 1200 c.c. 



The Vital Capacity of the Lungs. From foregoing statements it is 

 clear that the thorax and lungs are capable of a maximum degree of expan- 

 sion, at which moment the lungs contain their maximum volume of air. 

 This volume, whatever it may be, represents the entire capacity of the lungs 

 in the physiologic condition, and includes the tidal, the complemental, the 

 reserve, and the residual volumes. Mr. Hutchinson, however, defined the 

 vital or respiratory capacity of the lungs as the amount of air which can be 

 expelled by the most forcible expiration after the most forcible inspira- 

 tion, this therefore excludes the residual volume. The vital capacity 

 was supposed to be an indication of an individual's respiratory power, not 



FIG. 192. GAD'S PNEUMATOGRAPH. 



only in physiologic but also in pathologic conditions. Though averaging 

 about 230 cubic inches (3770 c.c.) for an individual 5 feet 7 inches in height, 

 the vital capacity varies with a number of conditions, the most important of 

 which is stature. It is found that between 5 and 6 feet the capacity in- 

 creases 8 inches (130 c.c.) for each inch increase in height. 



The Total Volume of Air Breathed Daily. For the solution of certain 

 problems connected with ventilation it is necessary to determine the total 

 volume of air taken nto the lungs in, the course of 24 hours. This can 

 be determined approximately if the two factors, the average volume of air 

 taken into the lungs at each inspiration, and the average number of res- 

 pirations per minute be known. If it be accepted that the inspired volume 

 varies from 330 to 500 c.c. and that the respiratory frequency averages 18 

 per minute, then the total volume breathed would amount to from 8500 to 

 12960 liters. 



The volume changes of the thorax indicated by the volumes of air en- 

 tering and leaving the lungs can be not only determined but graphically 

 represented by means of an apparatus similar in principle to the spirometer, 

 devised by Gad and known as the pneumato graph or aeroplethysmo graph 

 (Fig. 192). This consists of a quadrangular box with double walls, the 



