CHANGES IN AIR AND BLOOD IN RESPIRATION. 607 



per cent, of carbon dioxid. By the same mode of calculation the 

 oxygen in the alveoli, assuming that the expired air contains 16 

 per cent, and the nitrogen suffers no change, should be equal to 14 

 per cent, of an atmosphere or 106 nuns, of Hg (760 X 0.14). Actual 

 observations made by these authors upon human beings in which 

 the expired air was analyzed indicate that the composition of the 

 alveolar air may vary between the following limits: Oxygen be- 

 tween 11 and 17 per cent, of an atmosphere (83.6 to 129.2 mms. 

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

 (27.9 to 41.8 mms. Hg). 



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 air was then withdrawn by means of a suitable device and was 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

 (40.2 mms. Hg), and the C0 2 under a tension of 6 per cent. (45.6 

 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 83.6 to 129.2 mms. 27.9 to 41.8 mms. 



Membrane j 1 



f 



Venous blood . . . 40.2 mms. 45.6 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 (Pfluger). 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 

 diffusion of this gas it is probable that this difference suffices to 

 explain the exchange. In this matter one must bear in mind also 

 the very large expanse of surface offered by the lungs and the very 

 complete subdivision of the mass of blood in the capillaries. Thus, 



* Loewy and von Schrotter, " Zeitschrift fur experimentelle Pathologie 

 und Therapie," 1, 197, 1905. 



