RESPIRATION BEYOND THE LUNGS 393 



the combination of 2 with hemoglobin is greatly influenced by other 

 factors, and that it is these that are likely to be of physiological impor- 

 tance. 



In order to understand the conditions under which hemoglobin will 

 take up and give off 2 in the animal body, we must study the combining 

 power of hemoglobin when it is exposed to different partial pressures 

 of 2 (for laws governing this, see page 353). In the blood, the ex- 

 tremes of the partial pressure of 2 are represented, at the one end, by 

 that in the alveolar air, which we have seen to be about 100 mm. Hg, 

 and at the other, by that existing in the tissues, such as muscle, which 

 has been shown to be not more than 19 or 20 mm. Hg. We must further 

 bear in mind that the 2 in its passage from the alveolar air to the hemo- 

 globin and from the hemoglobin to the tissues, is transmitted in solution 

 through the plasma; that is, so far as the supply of 2 to the tissue cells 

 is concerned, the plasma serves as the immediate source. Since the tis- 

 sues are using up 2 at a very great speed, especially when active, and 

 are thus tending to lower the tension of 2 in the plasma, favorable con- 

 ditions have to be created whereby the hemoglobin liberates 2 at the 

 same rate as that at which it is leaving the plasma. In brief, it is the 

 2 tension of the plasma in the tissue capillaries that is the important 

 factor, the hemoglobin merely serving as a storehouse, which delivers 

 its 2 at just such a rate as to maintain the plasma-oxygen tension at 

 a constant level. It is obviously of the greatest importance that we 

 should understand how this mechanism of an adequate plasma-oxygen 

 tension is maintained. 



Methods of Investigation. We must remember that the combination 

 of 2 and hemoglobin, being a definite chemical reaction, will be re- 

 versible, and must, therefore, obey the laws of mass action (see page 

 23) according to the equation: Hb + 2 ^Hb0 2 . In order to ascertain 

 the position of the balance of this equation at different partial pressures 

 of 2 , that is, the relative quantities of oxy- and reduced hemoglobin 

 formed in a solution of hemoglobin when this is shaken with 2 at differ- 

 ent pressures, we may proceed as follows: A few c.c. of the hemoglobin 

 solution are placed in each of a series of vessels called tonometers, like 

 those shown in Fig. 134. In addition to the hemoglobin solution, each 

 tonometer contains a mixture of nitrogen and 2 in different propor- 

 tions. Suppose we use six vessels and in No. 1 have pure nitrogen; in 

 No. 2, nitrogen containing 5 mm. partial pressure of 2 ; in No. 3, 10 

 mm. ; in No. 4, 20 ; in No. 5, 50 ; and in No. 6, 100. We now rotate the 

 tonometers in a water-bath at body temperature for about twenty min- 

 utes, so that, by the formation of a thin film of hemoglobin solution over 

 the walls of the vessel, perfect equilibrium between the atmosphere and 



