THE CHEMISTRY OF RESPIRATION 503 



of this gas, while 100 c.c. of water under identical conditions are 

 capable of absorbing only 0.7 c.c. (0.7 volume per cent.). This fact, 

 that the oxygen is not simply absorbed by the blood, may also be 

 deduced from the observation that its quantity does not vary directly 

 with its partial pressure in the surrounding medium. It is a well- 

 known fact that blood exposed to the vacuum of an air-pump does not 

 discharge its oxygen until the pressure has been considerably reduced. 

 In most instances a diminution to about half an atmosphere is re- 

 quired before this gas begins to escape. This corresponds to a pressure 

 of oxygen of about 80 mm. Hg. At about 70 mm. Hg the dissociation 

 is intense, and becomes more and more rapid as the pressure declines 

 toward zero. Meanwhile, the blood changes its color from bright 

 red to purple. This behavior of the oxygen clearly proves that it is 

 not held in a simple physical condition, but enters into a dissociable 

 union with some constituent of the blood. 



If the blood is now centrifugalized, it will be found that the plasma 

 is capable of absorbing only a very small amount of oxygen, while 

 by far the greatest quantity of this gas is held in the corpuscular 

 elements. Only 0.65 volume per cent, are obtainable from the plasma. 

 Another striking difference is the variability of the oxygen content 

 of the plasma in consequence of changes in the tension of this gas in 

 the surrounding medium. If the latter is increased, a greater quan- 

 tity of oxygen will be absorbed by it, and vice versa. Consequently, 

 plasma behaves like water; i.e., it follows the Henry-Dalton law of 

 pressures absolutely. The corpuscular elements, on the other hand, 

 do not show a direct relationship of this kind. To be sure, they also 

 take up a greater amount of oxygen when the partial pressure of this 

 gas is high, but a more copious absorption takes place when its tension 

 is low. As higher degrees of pressure are reached, the absorption 

 becomes less, relatively speaking. 



This fact may be illustrated by subjecting defibrinated blood to different 

 tensions of oxygen. At the temperature of the body a pressure of 10 mm. led to 

 an absorption of 6 c.c. of oxygen, while 30 mm. of pressure sufficed for an absorp- 

 tion of more than 16 c.c. Consequently, these low tensions were sufficient to 

 produce a saturation of 80 per cent.; moreover, while higher pressures gave rise to 

 a still greater absorption, the increase obtained with each additional rise in pres- 

 sure, became gradually less. Thus, with 40 mm. of pressure only 2 c.c. were 

 taken up in addition to those already absorbed, and at 50 mm. only 1 c.c. It 

 has also been ascertained that the degree of saturation of the corpuscles which it is 

 possible to achieve with pure oxygen, namely, with a partial pressure of 760 mm. 

 Hg, is only slightly greater than that obtainable with atmospheric air in which this 

 gas exerts a pressure of only about 150 mm. Whole blood, on the other hand, 

 takes up a somewhat greater amount of oxygen if exposed to it in its pure form, 

 but this oxygen cannot be held by the corpuscles, because they are quite unable to 

 acquire much more than may be chemically united with them. Consequently, 

 this extra amount must be held by them in a physical state and must eventually 

 overflow into the plasma. It need scarcely be mentioned that oxygen thus dis- 

 solved in the plasma, obeys the ordinary laws of diffusion, i.e., it escapes from the 

 blood as soon as its partial pressure in the surrounding medium is diminished and 

 long before its chemically combined portion is liberated. These facts indicate 



