72 RESPIRATION 



On examining the dissociation curve it will be seen that the 

 steepest part of the curve is in the middle. In the case of oxy- 

 haemoglobin dissociating in the living body as the blood passes 

 through the capillaries, and in doing so taking up CO 2 , this part 

 of the curve is still steeper, for the reason given by Bohr and his 

 pupils. It is clear that with this form of curve the oxygen pressure 

 in the capillaries must tend, after the first fifth of the oxygen has 

 been given off, to remain comparatively steady during the giving 

 off of the next three-fifths : for at this stage a large amount of 

 oxygen is given off from the oxyhaemoglobin with a compara- 

 tively small fall in the oxygen pressure. In this way the oxygen 

 supply to the tissues is maintained at a far higher and also much 

 steadier pressure than if the curve were a rectangular hyperbola. 

 As will be seen later, a man would die on the spot of asphyxia if 

 the oxygen dissociation curve of his blood were suddenly altered 

 so as to assume the form which Hiifner supposed it to have in the 

 living body. The salts of the red corpuscles and the particular 

 hydrogen ion concentration of the blood are of essential impor- 

 tance in connection with the oxygen supply of the tissues. 



Haemoglobin, as already mentioned, forms specially colored 

 dissociable compounds, not only with oxygen, but also with carbon 

 monoxide and nitric oxide, and the compound with CO is of 

 special physiological interest, apart from its practical importance 

 in connection with the frequency of CO poisoning. As compared 

 with the oxygen compound the CO compound, which was dis- 

 covered by Claude Bernard, 23 is characterized by its relative 

 stability, which is so great that at one time it was supposed that 

 CO-haemoglobin is not dissociable. 



Blood of which the haemoglobin is saturated with CO has a 

 scarlet color similar to that of blood saturated with oxygen ; but if 

 the CO-haemoglobin is highly diluted, or examined in a very thin 

 layer, its color is pink, as compared with the yellow color of diluted 

 oxyhaemoglobin. By taking advantage of this fact one can easily 

 recognize the presence of CO-haemoglobin in blood. This test, as 

 I have often pointed out, is far more delicate than the older 

 spectroscopic test, but requires daylight or some similar light. By 

 adding carmine solution to diluted normal blood one can exactly 

 match the color of the diluted blood containing CO, 24 and by 

 using a suitable carmine solution I found it possible to estimate 



33 Claude Bernard, Compt. Rend., XLVIII, p. 393, 1858. 



"A detailed description of this method in its latest form will be found in 

 the Appendix. 



