RESPIRATION 



188 



carbonic acid from the air in bottles I and II, while the reverse has 

 taken place in bottles III, IV, and V. The carbonic acid tension of 

 the blood is therefore between 5'4 and 5'1 per cent. 



Relation of Tension to Composition. The aerotonometer may be 

 used to determine the relation of the tension of a gas in the blood 

 (oxygen or carbonic acid) to the quantity contained in that fluid. 

 The example given above might be extended for this purpose in the 

 following way. After the air has been withdrawn for analysis, it is 

 only necessary to attach A to the blood-bulb of a mercurial gas pump, 

 and, by forcing more water into B, expel a sufficient quantity of 

 blood for analysis. If an analysis was made for each of the bottles, 

 the quantity of carbonic acid would be determined corresponding to 

 the tensions, 5*8, 5'4, 5*1, 4'7, and 4'3 per cent, respectively. 



The comparision of the quantity of carbonic acid in the blood 

 with its tension is not of importance, but the corresponding measure- 

 ments for oxygen have been carried out with great care by a number 

 of observers, both for blood and for solutions of haemoglobin. 



It is found that the two sets of observations agree very closely, 

 and this fact forms the most substantial evidence for our belief that 

 the oxygen in the blood is almost entirely associated with the haemo- 

 globin. 



The relation of the quantity of oxygen in the blood to the tension 

 which it exerts may be conveniently set forth in a curve, which is 

 called the dissociation curve of oxyhaemoglobin. 



An example may illustrate 

 the use to which the informa- 

 tion, given in this curve, may 

 be put. The amount of blood 

 which comes, say, from the 

 pancreas is too small to allow 

 of a direct determination of the 

 tension of the oxygen which it 

 contains. We can, however, 

 calculate it from the following 

 data. (1) The amount of oxygen 

 which the blood would contain 

 if saturated (this can be arrived 



at by saturating arterial blood and analysing it). (2) The amount of 

 'oxygen in the venous blood. In an actual experiment (1) was 22'0 c.c., 



10*7 

 (2) was 10-7 c.c. The saturation then was 100 x ^^ = 49 per cent. 



a 



10 i 20 ?1 40 SO f.3 



i.0 03 ICO I!J 120 133 MO ISO 



Fm. 39. Dissociation of oxygen curves for blood (B) 

 and haemoglobin solution (H) at 38 C. The 

 figures along the base line are pressures in milli- 

 metres of mercury, and those along the ordinate 

 are percentages of oxygen. ( Bohr.) 



By referring to the dissociation curve we see that 49 per cent. 



