THE MECHANICS OF RESPIRATION 



329 



would be entirely venous. On the other hand, if the amount of air left 

 in the lungs at the end of expiration were above the normal amount, 

 each increment of C0 2 given off from the blood, or of 2 absorbed by 

 it would produce less change in the pressure of the C0 2 or 2 . 



Patients suffering from dyspnea, particularly those suffering from 

 cardiac dyspnea, can not breathe as comfortably when lying as when 

 sitting. This condition is known as orthopnea. The advantage of the sit- 

 ting over the lying position for breathing can not be satisfactorily ex- 

 plained. The greater vital capacity in the upright position; the favor- 

 ing of the return of the venous blood from the cerebral vessels by 

 gravity; the increased caliber of the pulmonary vessels because of the 

 enlarged thoracic cavity (see page 335) ; and the increase in the reserve 

 air of the lungs are all factors to be considered. 



The Vital Capacity. At one time it was thought that the vital capacity 

 of the lungs was related to their ventilatory capabilities, but for years 

 the determination of this value in patients has been considered unimpor- 

 tant. Eecently Peabody and Wentworth 10 have called attention to the 

 fact that patients with heart disease become dyspneic more readily than 

 do healthy subjects, and that this tendency seems to depend largely 

 on their inability to increase the depth of the respiration in a normal 

 manner. They find that this inability to breathe deeply corresponds to 

 a diminished vital capacity of the lungs, that is, the volume of the 

 greatest possible expiration after the deepest inspiration. They believe 

 that any condition which limits the possibility of increasing the minute 

 volume of air breathed must be an important factor in the production 

 of dyspnea. 



In normal adults the following averages (Table I), were secured from 

 a large series of clinical cases. The subjects are grouped into two 

 classes, each group being subdivided according to height. 



TABLE I 

 THE VITAL CAPACITY OF THE LUNGS of NORMAL MALES 



THE VITAL CAPACITY OF THE LUNGS OF NORMAL FEMALES 



(Peabody and Wentworth.) 



