CONDITION OF OXYGEN IN THE BLOOD 157 



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 ten- 

 sion 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 CO 2 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 oxygen 

 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 oxyhemoglobin. Only a 

 comparatively small part is held in solution by the plasma. Dis- 

 sociation of oxyhemoglobin occurs when the pressure is suffi- 

 ciently 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 arterialization. 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 CO 2 

 and the gain of O. In 100 volumes of arterial or venous blood 

 there are found to be, on an average, 60 volumes of O and CO 2 . 

 This total remains approximately constant, though the relative 

 amount of each gas varies according as the blood is venous or 

 arterial, and in venous blood under the influence of several con- 

 ditions to be mentioned. In arterial blood the O will represent 

 about 20, and the CO 2 about 40, of the total 60 volumes per 

 hundred of gas. In ordinary venous blood the O will represent 



