336 



RESPIRATION 



it is known to be chemically combined, Hoppe-Seyler showed that oxygen is 

 found exclusively in the red blood corpuscles combined with the haemoglobin. 

 Many investigations, some of them with haemoglobin solutions, some with 

 blood, were then made looking to a closer determination of the dependence 

 of oxygen absorption upon its partial pressure. It was not to be expected 

 a priori that the haemoglobin solutions would conduct themselves in exactly 

 the same way as the blood, for haemoglobin does not occur in the blood cor- 

 puscles as such, but in combination probably with lecithin. It appears from 

 these experiments that equal quantities of blood and haemoglobin combine 

 the same maximum quantities of oxygen, but at lower partial pressures the 

 two behave very differently. 



It is impossible to discuss here the facts bearing on the absorption of oxygen 

 by haemoglobin solutions of different concentrations and the theoretical conclu- 

 sions appertaining thereto. I shall limit myself therefore to the summarized 

 results obtained by Bohr with dog's blood, by Krogh with horse's blood (Fig. 

 131), and by Loewy with human blood, all at a temperature of 38. 



Oxygen Absorption in Percentage of Saturation 



Oxygen in small quantities is present also in the plasma. If all the oxygen 

 were to be removed from the plasma at once, dissociation of the oxyhsemoglobin 

 would of course take place immediately, and continue until equilibrium was 

 once more established between the oxygen tension in the plasma and in the blood 

 corpuscles. The coefficient of absorption of oxygen in the blood at body tem- 

 perature is approximately 0.025. 



Since the partial pressure of oxygen in the atmospheric air may be esti- 

 mated at about 160 mm. Hg., and in the alveoli, as we shall see later, at 

 120-130 mm. Hg., it follows that under normal circumstances the blood can 

 be saturated with oxygen up to ninety-eight per cent at least (Fig. 131). At 

 a partial pressure of 50 mm. Hg. the absorption of oxygen in man falls to 

 nineteen per cent of saturation, and in the dog to seven per cent. On the 

 other hand, the absorption is not noticeably greater in an atmosphere of 

 pure oxygen. 



These conclusions are confirmed by observations on respiration under dif- 

 ferent oxygen pressures. So far as absorption of oxygen is concerned, respira- 

 tion runs a perfectly even course when the partial pressure of oxygen is 

 raised from twenty-one to sixty, seventy-five, and ninety per cent. There is 

 an increase in the absorption only during the first three minutes of respira- 

 tion in air rich in oxygen, and this is due to the physical effect of a higher 



