686 PHYSIOLOGY OF RESPIRATION. 



directions until the pressure is equal on both sides. As the excess 

 of movement is from the point of higher pressure to the point of 

 lower pressure, attention is paid only to this side of the process, 

 and we say that the gas diffuses from a point of high tension to 

 one of lower tension. After equilibrium is established and the 

 pressure is the same on both sides we must imagine that the 

 diffusion is equal in both directions, and the condition is the same 

 as though there were no further diffusion. In order for this 

 theory to hold for the exchange in the body it must be shown that 

 the physical conditions are such as it demands. Numerous experi- 

 ments have been made, therefore, to determine the actual pressure 

 of the oxygen and carbon dioxid in the venous blood as com- 

 pared with the pressures of the same gases in the alveolar air, and 

 the pressures in the arterial blood as compared with those in the 

 tissues. Although the actual figures obtained have varied some- 

 what with the method used, the species or condition of the ani- 

 mal, yet, on the whole, the results tend to support the physical 

 theory. 



The Gaseous Exchange in the Lungs. It is difficult to deter- 

 mine the exact composition of the alveolar air. The expired 

 air can, of course, be collected and analyzed, but obviously this is a 

 mixture of the air in the bronchi and the alveoli, and consequently 

 has more oxygen and less carbon dioxid than the air in the alveoli. 

 The probable composition of the alveolar air has been calculated by 

 Zuntz and Loewy for normal quiet breathing in the following way : 

 The capacity of the 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 CO 2 , 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, A.e = ai -+- (A a) ,x, in which x represents the unknown 

 percentage of oxygen in the alveolar air. We have, therefore, x = A ~- 

 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 Jungs 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 



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

 iology," 47, 431, 1914. 



