CHEMISTRY OF RESPIRATION. 385 



in the alveolar air is about 114 mm., although one observer at 

 least places it as low as 99 mm. This has never been accurately de- 

 termined. The tension of CO 2 in the alveolar air is about 36 mm. 

 In order to ascertain why the O of the air goes to the blood and 

 the CO 2 of the blood to the air, we must first know the tension 

 of O and CO 2 in the blood. For this purpose an instrument 

 known as an aerotonometer is used. The principle underlying 

 this instrument is thus described by Pembrey in Schafer's Physi- 

 ology : " Blood in contact with a mixture of oxygen, nitrogen, and 

 carbon dioxid 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 equal to that of the corresponding gas in the mixture. 

 In the aerotonometer the blood is made to pass in a thin layer 

 through a glass tube or tubes containing mixtures of gases 

 of known quantity and tension, and it is arranged by prac- 

 tice that the tension of the gases shall in the one case be greater, 

 in the other case smaller, than the tensions of the corresponding 

 gases in the blood. The gases in these tubes, after the blood has 

 passed through them, are analyzed, and from the alteration in the 

 proportion in the two tubes it is possible to calculate the partial 

 pressure of the gases in the blood. The aerotonometer is sur- 

 rounded by a water-jacket with a temperature of 39 C." 



Another aerotonometer is that of Fredericq, and Bohr has 

 devised one known as an hemato-aerometen 



The results obtained by these different instruments vary con- 

 siderably. Strassburg gives the tension of CO 2 in venous blood 

 of the right side of a dog's heart as 5.4 per cent, of an atmosphere ; 

 and 2.2 to 3.8 per cent, in arterial blood. Herter gives the ten- 

 sion of O in arterial blood as 10 per cent, of an atmosphere. 

 Bohr has obtained quite different results: 101 to 104 mm. of 

 mercury for the tension of O in arterial blood higher than that 

 of the air in the trachea. He also found that when the dog, the 

 subject of the experiment, breathed pure air, the tension of the 

 CO 2 in arterial blood rises from nothing to 28 mm. of mercury, 

 and when the dog breathed air containing CO 2 the tension varied 

 between 0.9 and 57.8 mm. That is to say, the tension of CO 2 

 was greater in the tracheal air than in the blood. If this is so, 

 it is manifest that the passage of the CO 2 of the blood outward 

 to the air could not be due to diffusion ; so that to explain the 

 actual facts Bohr concludes that the tissues of the lungs play 

 an active part in the absorption of oxygen and the elimination of 

 carbon dioxid. Haldane and Lorraine Smith have substituted for 

 the aerotonometer a method by which " the tension of O in the 



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