RESPIRATION. 521 



animation and in hybernating animals they aid materially in pulmonary ven- 

 tilation. 



Besides these mechanical factors there is present the important factor of the 

 diffusion of gases, O diffusing toward the alveoli and CO 2 toward the anterior 

 nares. The rapidity with which diffusion occurs, other things being equal, 

 depends upon the differences in the " partial pressure " of the gas at various 

 regions. Each gas forming part of a mechanical mixture exerts a partial 

 pressure proportional to its percentage of the mixture. Thus, atmospheric air 

 contains 20.81 volumes per cent, of O, 0.04 volumes per cent, of CO 2 , and 79.15 

 volumes per cent, of N. If the air exists at 760 millimeters barometric pressure, 

 each gas will exert apart of the total pressure, or a " partial pressure," equivalent 

 to its respective volume. Should we wish to find the partial pressure of O, it 



.. 20.81 20.81 X 760 



may be ascertained simply by taking 01 the total pressure^- -. 



= 158.15 millimeters; similarly, the partial pressure of CO 2 would be 



0.04 X 760 - XT 79.15 X 760 



= 0.30 millimeter ; and that of N, -- ^. -- = 601.54 



100 



millimeters. Knowing, then, the composition of any mixture of gases and the 

 total pressure under which it exists, it is a matter of very simple calculation 

 to determine the partial pressure of each of the various gases constituting the 

 atmosphere. Expired air is poorer in O and richer in CO 2 than inspired air, 

 and alveolar air is altered even to a greater extent than expired air ; hence 

 the partial pressures must be affected similarly. 



The first portion of the air expired contains a maximum amount of inspired 

 air and a minimum amount of the air contained in the air-passages previous to 

 the inspiratory act ; but as expiration continues the mixture becomes poorer and 

 poorer in inspired air and similarly richer in the vitiated air from the smaller 

 air-passages and the alveoli ; in fact, the last portion of expired air is very 

 similar to, if not identical in its composition with, that in the alveoli. The 

 following partial pressures of O and CO 2 in inspired air and alveolar air 

 indicate the extent to which the composition varies in different parts of the 

 respiratory tract: 



Gaa. Inspired Air. Alveolar Air. 



O ............. 158.15 millimeters. 122 millimeters. 1 



CO, ............ 0.30 millimeter. 38 millimeters. 



Since the partial pressure of O in inspired air is about 158.15 millimeters, and 

 as it is but about 122 millimeters in the alveoli, and as the air is poorer in O as 

 we pass from the nares to the alveoli, it is obvious that a force must be exerted 

 constantly to cause a diffusion of O from the larger air-passages to the bron- 

 chioles and from the bronchioles to the alveoli that the O must diffuse 

 from the region of highest pressure to that of lowest pressure. During life 

 an equilibrium can never be established, because of the constant supply of 

 fresh air and the continual passage of O from the alveoli to the blood. The 

 1 The exact per cent, composition of alveolar air is not known ; these figures are estimates. 



