320 THE RESPIRATION 



no value in purifying the air already present in the alveoli. If we take 

 a tidal inspiration as amounting to 500 c.c. and the functional dead space 

 as 150 c.c., it is plain that only 350 c.c. of the outside air gains the 

 alveoli, and that the subsequent expiration is composed of 150 c.c. of 

 outside air that had lodged in the dead space plus 350 c.c. of alveolar air. 



These facts deserve a certain amount of emphasis because of their 

 practical importance in many phenomena connected with respiration. 

 One seldom thinks, for example, that out of the 500 c.c. of air inspired 

 with each breath, only 350 c.c. reaches the alveoli, where it comes in 

 contact with the 2500-3000 c.c. of air already present in this part of the 

 lungs. 



There must therefore be a sort of interface somewhere in the alveoli 

 between the fresh outside air that comes in with each breath through 

 the bronchioles and the air which is more or less stagnant in the alveoli. 

 This interface must move backward and forward somewhat with each 

 breath, and a rapid diffusion of oxygen and of C0 2 must take place 

 across it between the inspired air and that in the alveoli. It is impossible 

 to fix any anatomical point at which the interface occurs. 



The above described mechanism for the ventilation of the alveoli in- 

 sures the maintenance of slight but constant changes in the composition 

 of the air next the alveolar epithelium. It helps to prevent sudden varia- 

 tions in the amount of gases in the blood, particularly of C0 2 . Should 

 such variations occur, irregular stimulation of the respiratory and other 

 important centers that are influenced by the amount of this gas present 

 in simple solution in the blood, would be the result. The mechanism 

 serves as a sort of mechanical buffer by diminishing the sudden changes, 

 in gas concentration produced by inspiration and expiration. 



Respiratory Tracings 



The measurements of air for the determination of the foregoing val- 

 ues are made by the use of meters of various types. Sometimes, how- 

 ever, it is necessary to obtain an inscribed record of the respirations. 

 This may be either qualitative or quantitative. A qualitative record is 

 taken by attaching some sort of receiving tambour to the thoracic wall 

 (the best type is shown in Fig. 109), and connecting this with a record- 

 ing tambour arranged to write on a blackened surface. When it is 

 desired merely to count the respirations or to observe their regularity, 

 such a tracing is all that is required, but obviously it does not tell us 

 how much air has entered and left the lungs at each respiration. To 

 obtain a quantitative tracing, we must either connect a recording instru- 

 ment with the trachea or inclose the body of the animal in what is 

 known as a body plethysmograph. In observations on laboratory an- 



