RESPIRATION. 523 



vided that the pressure exerted by the gas in both cases be the same. Thus, 

 atmospheric air consists of 20.81 volumes per cent, of O, 0.04 volume per 

 cent, of CO 2 , and 79.15 volumes per cent, of N. Each gas exerts a partial 

 pressure in proportion to its percentage of the mixture. Assuming that the 

 air is at standard atmospheric pressure, the partial pressure of O is 20.81 per 

 cent, of 760 millimeters of Hg, or 158.15 millimeters. The quantity of O 

 absorbed from the air at C and 760 millimeters pressure is therefore the same 

 as when the atmosphere consists of pure O at a pressure of 158.15 millimeters. 



The absorption-coefficient must consequently be - 1 ' - = 0.01 vol- 

 ume. Therefore 100 volumes of water at C. and 760 millimeters pressure 

 absorb from the air 1 volume of O. 



If the partial pressure of O be increased or decreased, the quantity absorbed 

 will rise or fall accordingly. From this it is obvious that O must exist under 

 a certain degree of pressure to prevent its passing out of solution, which 

 is expressed by the term tension of solution, meaning, in a word, the pres- 

 sure required to keep the gas in solution. If the partial pressure of the gas 

 diminishes, the gas in solution is given off until the partial pressure of the 

 gas in the air and the tension of the gas in solution are equal. Conversely, as 

 the partial pressure of the gas in the air increases, the gas in solution will be 

 under correspondingly higher tension. 



Tension of 0. The absorption-coefficient of blood for O is nearly the same 

 as that of water, so that blood at should absorb from the atmosphere about 

 1 volume per cent, of O, but less than one-half as much at the temperature 

 of the body. The results of experiments show, however, that blood contains 

 considerably more than this, the average for arterial blood being 22.2 volumes 

 per cent., or very much more than can be accounted for by the laws of partial 

 pressures and tensions. Moreover, when the blood is subjected to a vacuum 

 pump there is evolved a small amount of gas consistent with the (diminution of 

 pressure, but the great bulk of it does not come off until the pressure has been 

 reduced to -$ to T ^- of an atmosphere. Finally, the quantity absorbed is 

 affected but little by changes in pressure above a certain standard. These 

 facts indicate that almost all of the O must be in chemical combination, the 

 combination being with haemoglobin in the form of oxyhaemoglobin. This 

 chemical union is readily dissociated at a constant minimal pressure which is 

 termed the tension of dissociation. There is a persistent tendency of the gas in 

 such a compound to become disengaged, so that when oxyhaemoglobin is placed 

 under circumstances where the tension or the partial pressure of O is less than 

 that in the compound, dissociation occurs ; conversely, when haemoglobin is 

 brought in contact with O at a pressure above the minimal constant of dissocia- 

 tion (^ to -j^ of an atmosphere), the two unite to form oxyha3moglobin. One 

 gram of haemoglobin combines, according to Hiifner, 1 with 1.59 cubic centimeters 

 of O at and 760 millimeters pressure. Assuming that 100 volumes of blood 

 contain 15 grams of hemoglobin (p. 335), if oxidized into oxyhaemoglobin the 

 1 Zeitschri/tfiirphysiologische Chemie, 1877-78, vol. ii. p. 389. 



