168 THE HUMAN MECHANISM 



oxyhemoglobin breaks up or dissociates into hemoglobin and 

 free oxygen. Applying this to the conditions in the capil- 

 laries, we find that 100 cc. of arterial blood contain less than 

 1 cc. of free oxygen in the plasma, but about 19 cc. of oxy- 

 gen combined in the oxyhemoglobin of the red corpuscles. 

 When the blood enters the capillaries of living tissues, oxy- 

 gen passes, as we have seen, from the plasma into the lymph, 

 so that the oxygen content of the plasma is reduced. When 

 this reduction goes below a point which is quickly reached, 

 dissociation of the oxyhemoglobin occurs, and the oxygen 

 thus set free in the plasma is drawn away by the lymph, 

 from which it is in turn drawn by the cell, the real seat 

 of oxidation. 



The amount of oxygen given above (20 cc. to 100 cc. of 

 blood) is all that the blood can hold under the usual con- 

 ditions of atmospheric pressure and at the temperature of the 

 body. Moreover, the oxygen content of the blood leaving 

 the lungs (arterial blood) is usually kept remarkably con- 

 stant by the accurate adjustment of the breathing movements 

 to the needs of the body. Neither by deeper nor by more 

 rapid breathing is it possible to increase appreciably the 

 amount of oxygen absorbed by the same volume of blood 

 flowing through the lungs. Only by increasing the quantity 

 of blood pumped through the lungs can we increase the 

 amount of oxygen carried to the organs and tissues; and, 

 for the same reason, only by increasing the quantity of 

 blood flowing through an organ can we increase the oxygen 

 supplied to that organ. 



6. The consumption of oxygen in the tissues. The quantity 

 of material oxidized in the cells of the body depends chiefly, 

 indeed under ordinary conditions of life it depends entirely, 

 on the amount of work these cells are doing. To put the 

 matter in another way, the cells always contain a certain 

 quantity of oxidizable material formed by the chemical 

 changes going on within them; during work, or activity, 



