74 RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



slowest of the partial reactions. Unless and until the reaction (or one 

 of the reactions) in which the molecular oxygen is involved is or be- 

 comes the slowest of the chain the oxygen supply will have very little 

 influence upon the aggregate reaction velocity. It will not be the 

 " limiting factor " in the sense defined by Blackmail [1910]. 



In attempts to determine the concentration at which oxygen be- 

 comes the limiting factor in the catabolic process the very serious 

 difficulty is encountered that the oxygen concentration which should 

 be determined is that of the cells in which the oxidations occur. 

 Extremely little is known so far about the oxygen concentration in 

 living tissues under normal conditions, and nothing whatever in cases 

 of oxygen want or with an increased supply of oxygen. In almost 

 all cases all that can be done is to determine the relation between the 

 oxygen pressure outside the organism and the rate of oxidation with- 

 in, but the information which can be gained in this way is slight only 

 and moreover very uncertain. 



It will be convenient to discuss separately the results obtained in 

 experiments on warm-blooded animals which, though far from being 

 satisfactory, are the least fragmentary. 



THE INFLUENCE OF A DIMINISHED OXYGEN PRESSURE ON THE 

 RESPIRATORY EXCHANGE OF WARM-BLOODED ANIMALS. 



Experiments on man and on several mammals (by Loewy [1895], 

 A. Loewy, J. Loewy and L. Zuntz [1897], Zuntz, Loewy, Miiller and 

 Caspari [1906], Hasselbalch and Lindhard [1914]) made in pneumatic 

 cabinets show that the total pressure can be diminished to about 

 400 mm. (about 80 mm. oxygen pressure) without any perceptible 

 influence upon the absorption of oxygen. At the lower pressures, 

 generally below 450 mm., the respiration is increased and some carbon 

 dioxide is washed out from the body, as seen from the high respiratory 

 quotients. 



In experiments on man, in which gas mixtures with a low percen- 

 tage of oxygen were breathed (Speck [1892], Loewy [1895], Durig 

 [1903]), the respiratory exchange, or standard metabolism, remained 

 constant until the inspired air contained 12 to 13 per cent, of oxygen 

 (90 mm.). By a further diminution of the oxygen percentage a slight 

 increase in oxygen absorption was observed which, however, when 

 corrected for the increased work of the respiratory muscles was con- 

 verted into a slight decrease. The differences between single ex- 



