CONDITION OF OXYGEN IN THE BLOOD 157 



greater than the tension of O in the blood ; consequently the 

 current is from the air to the blood. The tension of CO2 in 

 the venous blood is much greater than the partial pressure 

 of the CQz in the alveolar air; consequently the current is 

 from the blood to the air. 



But, here, as in the last analysis of almost all physiolog- 

 ical phenomena, it is found that, while these purely physical 

 laws are certainly concerned in the pulmonary interchange 

 of gases, they are insufficient to explain the occurrence in 

 full. For the blood will take from the alveolar air more than 

 enough O to establish an equilibrium of tension and partial 

 pressure; the tension of O in arterial blood is higher 

 than its partial pressure in alveolar air. So it is 

 found that the alveolar air will remove more than enough 

 CO2 to establish a similar equilibrium of this gas. It is 

 known that the avidity (chemical) of corpuscles for O to 

 form oxyhemoglobin causes the blood to appropriate more 

 O than it would otherwise do, but even then we are driven to 

 the usual ultimatum of ascribing some peculiar office to the 

 living epithelium of the intervening membrane. 



Condition of Oxygen in the Blood. Almost all the oxy- 

 gen is conveyed in the blood by the red corpuscles, where 

 it exists in rather unstable composition with hemoglobin 

 (probably with its pigment portion) under the name of oxy- 

 hemoglobin. Only a comparatively small part is held in so- 

 lution by the plasma. Dissociation of oxyhemoglobin oc- 

 curs when the pressure is sufficiently reduced. 



Alterations in Blood in Passing Through the Lungs. 

 The sum total of the changes taking place in the blood as it 

 passes through the lungs is represented by the term arteriali- 

 zation. In general, it may be said that the blood undergoes 

 changes exactly opposite to those of the air in circulating 

 through the pulmonary structure, and reference to the list 

 of substances gained and lost by the air will suggest the 

 main alterations in the blood. 



Of course the most striking phenomena are the loss of 



