xi EESPIEATOEY EXCHANGES 389 



is again in equilibrium witli the tension of the gases of the venous blood 

 circulating in it. The air is then aspirated from the sound, and its percentage 

 composition determined by Bunseii's method. 



The values found for the C0 2 and 2 tension indirectly indicate the 

 tension of these gases in the venous blood circulating in the capillaries of the 

 lungs. 



The tension of the 2 and CO 2 of arterial blood is determined by means 

 of the so-called aerotonometers. "The simplest form is that of Fredericq, 

 represented in Fig. 171. It consists of a glass tube, filled with a gaseous 

 mixture of known composition (10 per cent of 2 , 5 per cent C0 2 , and the 

 rest N), along the internal surface of which there is a constant flow of blood 

 from the carotid artery. During its passage through the tube, the tensions 

 of the blood-gases and of the artificial mixture of gases are equilibrated. 



By making the blood incoagulable through previous injection of pro- 

 peptone or albumose, and returning to the circulation by the jugular vein the 

 blood that left by the carotid, the experiment can be prolonged for a 

 considerable time (an hour or more), so as to be certain of having established 

 equilibrium of tension between the gases of the arterial blood and those 

 contained in the aerotonometer. The blood that flows through the instrument 

 is maintained at body temperature by means of an external glass jacket in 

 which water is circulated at the required temperature. When the experiment 

 is completed, the percentage composition of the mixture of gases in the 

 aerotonometer is calculated by the usual method, and the values obtained 

 express the partial pressure of the 2 and C0 2 of arterial blood. 



The experiments on the tension of the respiratory gases have 

 not led to uniform results : they vary greatly even in the same 

 animals under slightly different conditions. 



The following table gives the average values cited by F. 

 Schenck and A. Giirber in their Text-Book of Human Physiology 

 (1897), which all relate to experiments carried out on dogs: 



These data coincide perfectly with the theory which holds 

 the respiratory gas exchanges to be the effects of simple 

 diffusion, which causes the gases to pass from the point of greater 

 to that of less tension. In fact the tension of 2 is seen to 

 diminish from inspired to expired air, and from that to arterial 

 blood; and the tension of C0 2 diminishes from venous blood to 

 that of expired air, and thence to that of inspired air. The 

 oxygen must therefore be absorbed by diffusion from the respiratory 

 passages into the arterial blood, while the carbonic acid must be 

 exhaled by diffusion from the venous blood into the respiratory 

 passages. 



The results of Bohr's subsequent experiments do not, however, 

 agree with this theory. With one of his special aerotonometers 



