308 RESPIRATION 



toward the lower and in the direction indicated by the arrows in the table 

 below. As fast as the oxygen diffuses into the venous blood, thus tending to 

 raise the pressure of the gas in solution, it is taken up and fixed by the hemo- 

 globin. This process proceeds far enough during the interval the blood is 

 in the pulmonary capillaries to raise the oxygen tension from 22.04 mm. 

 of mercury to 29 . 64 mm. of mercury, and also far enough to permit of the 

 fixation of from four to five volumes per cent, of oxygen. The oxygen 

 diffusion pressures are indicated as follows: 



Oxygen pressure in the atmosphere 2 1 per cent, or 1 59 mm. of mercury 



I 



Oxygen pressure in the alveolar air 16 per cent, or 122 mm. of mercury 



1 



Oxygen pressure in the venous blood 3 per cent, or 22 .04 mm. of mercury 



Liberation of Oxygen in the Tissue Capillaries. When the arterial 

 blood reaches the capillaries of the tissues, then the situation which we have 

 just found holding good in the lungs is reversed. As rapidly as the oxygen 

 reaches the living protoplasm of the tissues it enters into fixed combination, 

 thus rendering it inert. The oxygen tension in the tissue cells will, there- 

 fore, be zero. Under these conditions the difference in pressure level be- 

 tween the oxygen tension in the blood and that in the tissues is sufficient to 

 cause a rapid diffusion of oxygen through the capillary walls with correspond- 

 ing liberation of the oxygen from the hemoglobin according to the laws of 

 combination given in the curves above. The total effect of this process is to 

 maintain a relatively high and constant diffusion pressure of the oxygen in 

 the blood. During the time the blood remains in the capillaries the total 

 oxygen tension will have been lowered from 29 .64 to 22 .04 mm. of mercury, 

 yet this slight lowering of tension is sufficient to liberate from four to five 

 volumes per cent of oxygen. This figure, of course, is comparative. In 

 many of the very active tissues, such as in muscle, a much larger per cent, 

 of oxygen will have been dissociated and the oxygen tension correspondingly 

 lowered so that the venous blood returning through such an active organ 

 may not have more than half the average amount of oxygen found in venous 

 blood. 



Considering the pressure relations of oxygen from the time of its intro- 

 duction into the body with the fresh air to its fixation in the tissues we have 

 the following schema: 



Oxygen pressure in the atmosphere 1 59 mm. 



I 



Oxygen pressure in the alveolar air 122 mm. 



I 



Oxygen pressure in the venous blood 22 .04 mm. 



Tension of oxygen in the arterial blood 29 . 64 mm. 



! 



Tension of oxygen in the tissues o . oo mm. 



