588 THE GASES OF THE BLOOD 



bined in the haemoglobin of the blood. The solvent capacity 

 of the plasma for oxygen does not appreciably differ from 

 that of water. In spite of the fact that the plasma there- 

 fore contains relatively little oxygen, this particular frac- 

 tion of the oxygen is in precise consideration of the greater 

 importance, for the tissue cells do not come in direct contact 

 with the oxygen-bearing red blood corpuscles, but with the 

 plasma only. The erythrocytes serve as a reservoir from 

 which the plasma-oxygen is continuously supplied. 



Tension Curves. The intake of oxygen into the blood 

 naturally depends to a very marked degree upon the oxygen 

 pressure in the air. For each given pressure there cor- 

 responds a definite amount of fixed oxygen. By taking the 

 pressures as abscissas and the corresponding amounts of 

 oxygen taken into the blood as ordinates, a tension curve 

 may be constructed. The course of the curve indicates how 

 much oxygen is combined by the haemoglobin at a given 

 pressure. If the total amount of oxygen in a unit volume 

 of blood be known for this particular moment, it is possible 

 to come to a conclusion as to the amount which is fixed by 

 the affinities of haemoglobin and how much is available in 

 free state. 



The general type of such tension curves is always the 

 same in all sorts of blood and haemoglobin solutions. In- 

 variably the line is that of a curve with its concavity toward 

 the axis of the abscissa, sharply ascending and then continu- 

 ing asymptomatic ally, this being expression of the fact 

 that the fixed amount of gas, as the pressure is gradu- 

 ally raised, increases far more quickly at a slight pressure 

 than at higher pressure. Even at an oxygen pressure of 

 160 mm. which about corresponds to the partial pressure 

 normal for the atmospheric oxygen, the haemoglobin seems 

 to be within a slight percentage of saturation with oxygen. 

 The maximal capacity for oxygen fixation both of fresh blood 

 and of haemoglobin solutions prepared by different methods 

 has been found by Hiifner, and a number of other inves- 



