430 



LECTURE XVIII. 



of an atmosphere. The corresponding value for the blood of the right 

 heart was 3.81 per cent. 



According to the results of this experiment, it would seem that we were 

 unquestionably justified in assuming that the gas-exchange in the alveoli 

 of the lungs takes place in accordance with the well-known laws of gas- 

 diffusion. Quite recently, however, and especially by the extended experi- 

 ments of Bohr, 1 facts have become known which cannot be explained on 

 this basis. Bohr desired, in each experiment, to know the composition of 

 the alveolar air. A good idea of this can be obtained by analyzing the 

 out-going air, obtained at the moment it passes the bifurcation of the 

 trachea. Such air contains more oxygen and less carbon dioxide than 

 does alveolar air, but, on the other hand, it contains less oxygen and more 

 carbon dioxide than the expired air; it represents a mean between the two. 

 It is essential in such experiments that the gas tension of the arterial 

 blood should be ascertained at the same time and with the same individual. 

 Bohr experimented with large dogs which he compelled to breathe through 

 easily movable valves. A gas-meter measured the amount of the expired 

 air, from which a sample was taken for analysis. Bohr noted the depth 

 of each inspiration and determined, after the death of the animal, the 

 volume of the trachea and the bronchial tubes. From these values he 

 computed the composition of the air at the bifurcation. The partial pressure 

 of the oxygen and carbon dioxide in this gas was thereby known. Simul- 

 taneously, the gas-pressure from the blood of an artery was measured, in 

 order to establish normal relations as far as possible. Bohr prevented 

 coagulation of the blood by injecting peptone solution, or leech extract, 

 and carried the blood back through a vein into the general circulation, 

 so that the result of the experiment was not influenced by loss of 

 blood. 



The results of these experiments showed, on the one hand, that the 

 oxygen tension of the arterial blood flowing out of the lungs is frequently 

 more than that of the air at the bifurcation and, on the other hand, in 



1 Skand. Arch. Physiol. 2, 236 (1891). 



