CAUSES OF THE EXCHANGE OF GASES. 



775 



The force with which the oxygen separates from the haemoglobin 

 under these circumstances is called the tension of dissociation. The 

 most important researches upon this subject are those of Htifner. 1 



The conditions of the dissociation of oxyhsemoglobin are the same, whether 

 it is a solution of freshly-made pure crystals of haemoglobin, or fresh 

 defibrinated blood. The dissociation is dependent upon the concentration of the 

 solution of haemoglobin ; thus, a weak solution is more readily dissociated under 

 a given pressure than a strong solution. It is also affected by temperature. 2 

 As regards pressure, Htifner found in the case of a solution containing 14 per 

 cent of oxylisemoglobin at 35, that, under a tension of oxygen of 152mm., 

 98*42 per cent, of the pigment was oxy haemoglobin, and 1*58 per cent, 

 haemoglobin. When the tension of oxygen was reduced to 75 mm., the 

 percentages of oxyhaemogiobin and of haemoglobin were respectively 96*89 and 

 3 '11, and with a lower pressure the dissociation became more rapid, as shown 

 by the following curves : 



/6O 



FIG. 72. Curves of dissociation of oxy hemoglobin. The continuous line is for a 

 solution containing 14 per cent, of haemoglobin, the interrupted line for a 

 4 per cent, solution. 



It is now necessary to compare with the tension of the oxygen and 

 carbon dioxide in the alveolar air the tension of those gases in the blood. 

 For the determination of these tensions in blood Pfliiger' 3 used a special 

 instrument, known as the aerotonometer (see Fig. 73). 



The principle of the aerotonometer and of other similar instruments is this : 

 Blood in contact with a mixture of oxygen, nitrogen, and carbon dioxide gives 

 up some of its gases if their partial pressures are greater than those of the 

 corresponding gases in the mixture ; on the other hand, if the tensions of the 

 gases in the blood be lower than the respective tensions of the gases in the 

 mixture, the blood takes up gas. Those interchanges persist until equilibrium 

 is established, until the tension or partial pressure of the gas in the blood is 



1 Ztschr. f. physiol. Chem., Strassburg, Bd. vi. S. 109; Bd. xii. S. 582; Bd. xiii. 

 S. 285 ; Arch. f. Physiol., Leipzig, 1890, S. 1 ; ibid., 1895, S. 213. 



2 Brasse, Compt. rend. Soc. de biol., Paris, 1888, S. 660. 



3 Described by Strassburg, Arch. f. d. gcs. Phy-nol., Bonn, 1872, Bd. vi. S. 65. 



