270 RESPIRATION 



of these two observers show that when the oxygen tension in the air, which is in 

 contact with the blood, is lowered below a certain point, the amount of oxygen 

 which will be liberated from combination with hemoglobin will be very great, 

 whereas a lowering of the tension of oxygen by an equal amount where the 

 pressures are relatively high leads to practically no liberation of hemoglobin, 

 and the converse is equally true. The critical oxygen pressure in so far as 

 its combination with hemoglobin is concerned varies according to observers. 

 With Loewy the critical dissociation pressure is at or below 76 mm. of mer- 

 cury, 10 per cent of an atmosphere. Strassburg gives the oxygen tension of 

 arterial blood as 29.64 mm. of mercury, and for venous blood 22.04 mm. of 

 mercury. That is to say, during the brief interval in which the blood is 

 in the pulmonary capillaries the oxygen tension has increased by 7.6 mm. 

 of mercury, an increase of tension which would produce very little increase 

 in simple absorption of oxygen. Yet is it sufficient to cause fixation of 

 from four to five volumes per cent of oxygen by the hemoglobin. 



Oxygen Pressure in the Atmosphere 159 mm. of mercury 



" " " Alveolar air 122 mm. of mercury 



i 



" " " Venous blood 22.04 mm. of mercury 



It is evident that there will be diffusion of oxygen from the high tension 

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

 above. 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 during the interval the blood is fh the pul- 

 monary capillaries far enough to raise the oxygen tension from 22.04 mm. 

 of mercury to 29.64 mm. of mercury, and far enough to permit of the fixation 

 of from four to five volumes per cent of oxygen. 



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 table above. The total effect of this process is to 

 maintain a relatively high 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 



