240 A MANUAL OF PHYSIOLOGY 



automatic arrangement by which oxidation is favoured. 

 Whatever may be thought of this view and objections to 

 it are not wanting the current theory, that the corpuscles 

 are simply passive carriers of oxygen, and exercise no further 

 influence on the plasma, breaks down in face of the facts. 

 We must admit that an active and many-sided commerce 

 exists between them and the liquid in which they float. 

 The nitrogen of the blood is simply absorbed. 



The Tension of the Blood-gases. If the gases of the blood existed 

 in simple solution, their tension or partial pressure could be deduced 

 from the amount dissolved and the co-efficient of absorption. Since 

 they are chemically combined, it is necessary to determine it directly. 

 This has been done by means of an apparatus called the aerotono- 

 meter (Pfliiger, Strassburg). The blood is made to pass directly from 

 the vessel to two tubes, which it traverses at the same time, the 

 stream being divided between them ; it then passes out again. The 

 tubes are warmed by means of a water-jacket to the body-temperature. 

 One of them is filled with a gaseous mixture having a greater, and 

 the other with a mixture having a smaller, partial pressure, say of 

 carbon dioxide, than is expected to be found in the blood. As the 

 latter runs in a thin sheet over the walls of the tubes, it loses carbon 

 dioxide to the one and takes up carbon dioxide from the other. 

 From the alteration in the proportion of the carbon dioxide in the 

 two tubes, it is easy to calculate the partial pressure of that gas in 

 the blood ; that is, the partial pressure which it would be necessary 

 to have in the tubes in order that the blood might pass through them 

 without losing or gaining carbon dioxide (p. 232). 



The pressure of oxygen in arterial blood was given by 

 Strassburg as about 30 mm. of mercury in the dog, and in 

 venous blood as something like 20 mm. If we were to 

 accept the recent experiments of Bohr, made by means of a 

 special form of aerotonometer constructed and worked much 

 in the same way as Lud wig's stromuhr (p. TIO), and inserted 

 into the course of a bloodvessel, it would be necessary to 

 treble or quadruple these numbers. 



The pressure of carbon dioxide in arterial blood we may 

 take at 10 to 40 mm., in venous blood at 30 to 50 mm., 

 according to the results of different observers. 



Whenever the venous blood has to pass through a region 

 in which the pressure of carbon dioxide is kept lower than 

 in itself, it will begin to lose carbon dioxide by diffusion. 

 If the pressure of oxygen in this region is at the same time 



