BESPIRA TION. 41 5 



whether the gas exist free or as a constituent of a complex atmosphere, pro- 

 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 C0 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 0° C and 760 millimeters pressure is therefore the same 

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



ti i m • , .11 20.81 X 0.0489 _ .. _ 



Ihe absorption-coemcieut must consequently be - — — = 0.01 vol- 



ume. Therefore 100 volumes of water at 0° 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 O. — The absorption-coefficient of blood for O is nearly the same 

 as that of water, so that blood at 0° 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 (see table, p. 411), and 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 greal bulk 

 of it does not come off until the pressure has been reduced to -fa to -,- 1 ,, of an 

 atmosphere. Finally, the quantity absorbed is affected but little by changes 

 in pressure above or below a certain standard. These facts indicate that 

 almost all of the O must be in chemical combination. This combination Is 

 with haemoglobin in the form of oxyhemoglobin. This chemical union i> 

 readily dissociated at a constant minimal pressure which is termed the tension 

 of dissociation. There is a persistenl tendency of the gas in such a compound 

 to become disengaged, so that when oxyhemoglobin is placed under circum- 

 stances where the tension or the partial pressure of <> is less than that in 

 the compound dissociation occurs ; conversely, when haemoglobin is brought 

 in contact with Oat a pressure above the minimal constanl of dissociation 

 (..'.. to ] \ t of an atmosphere), the two unite to form oxyhemoglobin. One 

 gram of haemoglobin from ox blood combines, according to Kufner,' with 

 1.34 cubic centimeters of O at 0° and 760 millimeters pressure. Assuming 

 1 Arehiv fiir Anatomie und Physiologie, 1894, S. L30, 



