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 Hiifner. 1 
The conditions of the dissociation of oxyhaenioglobin 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, Hiifner found in the case of a solution containing 14 per 
cent of oxyhemoglobin at 35°, that, under a tension of oxygen of 152 mm., 
98"42 per cent, of the pigment was oxyhemoglobin, and 1*58 per cent, 
haemoglobin. When the tension of oxygen was reduced to 75 mm., the 
percentages of oxyhemoglobin and of haemoglobin were respectively 96"89 and 
311, and with a lower pressure the dissociation became more rapid, as shown 
by the following curves : — 
so 
70 
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Fig. 72. — Curves of dissociation of oxyhemoglobin. 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 '■'• 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. These interchanges persist until equilibrium 
is established, until the tension or partial pressure of the gas in the blood is 
l 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. 
- Biasse, Compt. rend. S'oc. de biol., Paris, 1S8S, S. 660. 
3 Described by Strassburg, Arch. f. d. ges. Thynol., Bonn, 1872, Bd. vi. S, 65. 
