METHOD FOR DETERMINING RESPIRATORY EXCHANGE IN MAN 563 



Determination of E.Q. 



O 2 in atmospheric air 20.94% 



O 2 - CO 2 in expired air (1(5.50 + 4) = 20.50% 



100-20.94 = 79.06%," N in atmospheric air. 

 100-20.50 = 79.50%, N in expired air. 



Since the nitrogen is not changed in volume, the last figure shows that 

 more oxygen must have been taken in during inspiration than 2 + C0 2 

 has been given back in expiration. This obviously must 'be taken into 

 account in the calculations. The amount of 2 actually inspired for each 

 100 c.c. of air expired is found as follows: 



20.94 (% O 2 in atmospheric air) 



79.06 (% N 2 in atmospheric air) X 79 ' 50 <% N ' in ex P ired air ) > or ' 265 ( con 

 stant factor X 79.5 (% N found for this observation) =21.07, the volume of O 2 which 

 would have been present in expired air to account for N present. t 

 21.07-16.50 = 4.57% O 2 actually absorbed. 

 4.00 - 0.03 (CO 2 in inspired air) = 3.97% CO 2 excreted. 



3.97 



' 0.87, the respiratory quotient, or ratio of CO, excreted to O., absorbed. 



Total Gas Exchange. The volume of air expired in 15 minutes into 

 the Tissot spirometer was found to be 100 liters measured at 20 C. and 

 747 mm. Hg (brass-scale barometer). This volume of gas must be cor- 

 rected so as to give the volume of dry air at and 760 mm. Hg. To do 

 this two things must be taken into account. (1) Since the expired air is 

 saturated with water, the pressure due to water vapor must be subtracted 

 from the observed barometric pressure to obtain the true pressure. The 

 vapor tension of water for various temperatures is given in Table II 

 on page 564.. (2) The barometer tube lengthens, or contracts with heat 

 or cold, and therefore the barometric readings must be corrected. 

 The corrections for ordinary barometric readings are found in Table III, 

 page 565. The figure corresponding to the temperatures is subtracted 

 from the barometric reading in order to obtain correct barometric pres- 

 sure. 



In the above experiment, the correction for the barometer is 2.41 mm. 

 (see Table III, page 565), and that for vapor tension at 20 C. is 17.4 

 (see Table II, page 564). 



Actual Barometric Pressure. 747 - (17.5 + 2.39) = 727.21 mm. The 

 coefficient of expansion of gases is taken as 0.003665) or 1/273 ; therefore 

 the volume of equals the volume at 1 divided by 1-0.003665 t; and 

 hence 



"This is the constant O percentage :n air. 



tThis calculation can be simplified by using an abbreviated table (page 564) giving the O figure 

 corresponding to the various percentages of N in the expired air. 



