678 PHYSIOLOGY OF RESPIRATION. 



The capacity of th^ bronchial tree is 140 c.c, and this air may be 

 considered as similar in composition to atmospheric air, that is, the 

 inspired air. A normal expiration contains 500 c.c; hence the. 

 alveolar air constitutes only 360 c.c. or if of the entire amount. If 

 the expired air contains 4.38 per cent, of CO2, then the alveolar 

 air must contain 4.38 ^ if, or 6 per cent, of carbon dioxid.* 



Or, to put the mode of calculation in a more general form, the amount 

 of oxygen in the expired air is equal to the amount of oxygen in the true 

 alveolar portion of the expired air plus the amount of oxygen in the "dead 

 space, " namely, the trachea and bronchi. Let A equal the volume of expired 

 air, e the percentage of oxygen in the expired air, a the volume of air in the 

 dead space, and i the percentage of oxygen in this air or what is the same 

 thing in the inspired air. According to the above statement we have the fol- 

 lowing equation, Ae ^ ai + (A — a) x, in which x represents the unknown 

 percentage of oxygen in the alveolar air. We have, therefore, x = 'f~z^^- 

 In ordinary breathing these values are as follows: A -- 500 c.c, a =■ 140 c.c, 

 e = 16.02 per cent., and i = 20.96 per cent. Substituting these values, x will be 

 found equal to 14.1 per cent. Reckoned in millimeters of mercury this would 

 be equal to (760 X 0.141) 107.2 mm. In order, however, to ascertain the 

 true pressure exerted by the oxygen allowance must be made for the baro- 

 metric pressure and for the tension of the aqueous vapor. In the depths of 

 the lungs the air is saturated with water vapor and the tension of this vapor 

 at the body temperature may be valued at 46.6 mms. Hg. If we suppose 

 further that the observation was made at a barometric pressure of 760 mms., 

 then the pressure of the oxygen in the alveoli would be (760 — 46.6X0.147) 

 99+ mms. Hg. 



Actual observations made by these authors upon human beings 

 in whom the expired air was analyzed indicate that the composition 

 of the alveolar air may vary under different conditions between 

 the following limits: Oxygen between 11 and 17 per cent, of an 

 atmosphere; carbon dioxid between 3.7 and 5.5 per cent, of an 

 atmosphere. Haldane and Prie-stley have devised a simple 

 method by means of which the last portions of the air breathed 

 out in an expiration may be collected. The sample thus collected 

 represents practically the alveolar air, and its average composition 

 for normal quiet respirations may be given as oxygen, 14 per cent, or 

 100 mms. Hg. (760 — 46.6X0.14); carbon dioxid, 5.5 per cent, or 

 40 mms. Hg. ; nitrogen, 80 per cent, or 570 mms. Hg. Other ob- 

 servers state that the direct determination of the CO2 in the alveolar 

 air by the method of Haldane and Priestley gives figures that are 

 too high, and that it is safer to estimate this factor indirectly. 

 Pearcef suggests the following method : The expired air from two 

 expirations of different depths is collected, measured exactly, and 

 analyzed for its CO2. With these factors known, and representing 

 the percentage of CO2 in the alveolar air by ?/, and the volume of 



* For discussion of methods, see Krogh and Lindhard, "Journal of Phys- 

 iology," 47, 431, 1914. 



t Pearce, "American Journal of Physiology," 43, 73, 1917. 



