682 PHYSIOLOGY OF RESPIRATION. 



further that the observation was made at a barometric pressure of 750 mms., 

 then the pressure of the oxygen in the alveoli would be (750 46.6X0.141) 

 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 Priestley 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, 14.5 per 

 cent.; carbon dioxid, 5.5 per cent.; and nitrogen, 80 per cent. 



Loewy and von Schrotter have determined also the average 

 tension of these gases in the blood of man. Their method* con- 

 sisted 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 repre- 

 sented 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 A portion of this air was then withdrawn by means of a 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 the alveolar atmosphere (710 X .053 = 37.6 mms. Hg.), 

 and the CC>2 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 to 40 mms. 



Membrane j + 



f I 



Venous blood . . . 37.6 mms. 42.6 mms. 



Diffusion must take place, therefore, in the direction indicated 

 by the arrows. As the oxygen passes through into the blood it is 

 combined with the hemoglobin and it is estimated that the arterial 

 blood as it flows away from the lungs is nearly saturated with 

 oxygen, lacking perhaps only 1 volume per cent, of being completely 

 saturated (Pfltiger). That is, if the normal arterial blood contains 

 19 c.c. of oxygen for each 100 c.c. of blood, it is probable that one 

 more cubic centimeter might be combined by the hemoglobin if 

 exposed fully to the air or oxygen. The difference in tension 

 between the carbon dioxid on the two sides of the membrane is not 

 so great as in the case of the oxygen, but owing to the more rapid 



* Loewy and von Schrotter, "Zeitschrift fur experimentelle Pathologic 

 und Therapie," 1, 197, 1905. See also Loewy, "Handbuch der Biochemie," 

 IV 1 , 1908. 



