628 PHYSIOLOGY OF RESPIRATION. 



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 -|-f of the entire amount. If 

 the expired air contains 4.38 per cent, of CO 2 , then the alveolar 

 air must contain 4.38 -*- 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 -f (A a) x, in which x represents the unknown 



percentage of oxygen in the alveolar air. We have, therefore, x = "^5?^*- 

 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 tension 

 of the aqueous vapor. At the temperature of the body this is estimated at 

 50 mm., hence the corrected atmospheric pressure in the alveoli will be 760 50 

 or 710 mm., and this multiplied by 0.141 gives 100 mm. as the pressure of 

 the oxygen in the alveolar air. 



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 between the following limits: Oxygen 

 between 11 and 17 per cent, of an atmosphere; carbon dioxid be- 

 tween 3.7 and 5.5 per cent, of an atmosphere. 



Loewy and von Schrotter have determined also the average ten- 

 sion of these gases in the blood of man. Their method * consisted 

 in blocking off one lung or one lobe of a lung by a metal catheter 

 inserted through the trachea. After the lapse of half an hour or 

 so the gases in this occluded portion had reached an equilibrium 

 by interchange with the venous blood which represented the tension 

 actually existing in the circulating venous blood. A portion of this 

 air was then withdrawn by means of a suitable device and was 

 analyzed. Their average result was that in the venous blood the 

 oxygen exists under a tension of 5.3 per cent, of an atmosphere 

 (710 X .053 = 37.6 mms. Hg), and the Co, under a tension of 6 

 per cent. (42.6 mms. Hg). The physical relations of pressure 

 between the alveolar air and the gases in the venous blood may be 

 represented as follows : 



OXYGEN. CARBON DIOXID. 



Alveolar air 100 mms. 35 mms. 



Membrane "-. 



t I 



Venous blood ... 37.6 mms. 42.6 mms. 



* Loewy and von Schrotter, " Zeitschrift fur experimentelle Pathologie 

 und Therapie," 1, 197, 1905. 



