338 MANUAL OF PHYSIOLOGY. 



ently remains in the lungs after they have collapsed. This 

 amounts to about 2000 cc. (or about 120 cubic inches.) 



Vital capacity is a term given to the greatest amount of air that 

 can be emitted by a forced expiration immediately following a 

 forced inspiration, so that it equals the sum of the tidal, reserve, 

 and complemental air. The vital capacity is estimated by spiro- 

 meters of different kinds, and gives an approximate measurement 

 of (1) the capacity of the chest; (2) the power of the respiratory 

 muscles ; (3) the resistance offered by the elasticity or rigidity of 

 the walls of the thorax ; (4) the working capacity of the lungs, 

 i.e., their extensibility or freedom from disease. It, therefore, 

 varies greatly according to the age, sex, position of the body, the 

 occupation, weight, height, the fulness of the hollow viscera of 

 the abdomen, and the pathological condition of the lungs. It can 

 be much increased by practice, and this fact, apart from the 

 injury forced respirations may produce in a morbid state of the 

 lung, renders it inapplicable as a gauge of pulmonary disease. 



From the foregoing it appears that the volume of air habitu- 

 ally sojourning in the lungs during natural respiration, or station- 

 ary air, is about 3500 cc. (225 cubic inches), while the fresh air 

 introduced by each inspiration is only a little over 500 cc. (30 

 cubic inches), or, in other words, about one-seventh of the air in 

 the lungs is changed at each breath. Indeed, the 500 cc. of 

 air is only just sufficient to fill the trachea and larger bronchial 

 passages, so that the fresh air does not reach the pulmonary 

 alveoli, or directly replace any of the air they contain. The tidal 

 stream is, however, brought into immediate relation with the sta- 

 tionary air, and the thoracic movements cause them to mix me- 

 chanically, so that rapid diffusion takes place in the minute 

 bronchi. Diffusion is also constantly occurring between the air 

 of the small tubes and the terminal sacs, and it alone suffices to 

 maintain the necessary standard of purity in the air of the alveoli. 

 If, during breathing, a harmless gas, such as hydrogen, be in- 

 haled during one inspiration, it requires 6 to 10 respirations to 

 get rid of the impurity from the expired air. From this it has 

 been inferred that this number of respiratory acts would be neces- 

 sary to render the air in the alveoli quite pure even if no fresh 

 impurities were allowed to enter from the blood. 



