PASSAGE OF OXYGEN THROUGH THE BODY 109 



Passage of Oxygen through the Blood. In serving its 

 purpose at the cells, the oxygen passes twice through the 

 blood once as it goes toward the cells and again as it 

 passes from the cells to the exterior of the body : 



Passage toward the Cells. This is effected mainly 

 through the hemoglobin of the red corpuscles. At the 

 lungs the oxygen and the hemoglobin form a weak chemi- 

 cal compound that breaks up and liberates the oxygen when 

 it reaches the capillaries in the tissues. The separation 

 of the oxygen from the hemoglobin at the tissues appears 

 to be due to two causes : first, to the weakness of the 

 chemical attraction between the atoms of oxygen and the 

 atoms that make up the hemoglobin molecule ; and second, 

 to a difference in the so-called oxygeti pressure at the lungs 

 and at the tissues. 1 



The attraction of the oxygen and the hemoglobin is suf- 

 ficient to cause them to unite where the oxygen pressure is 

 more than one half pound to the square inch, but it is not 

 sufficiently strong to cause them to unite or to prevent 

 their separation, if already united, where the oxygen pres- 

 sure is less than one half pound to the square inch. The 

 oxygen pressure at the lungs, which amounts to nearly 

 three pounds to the square inch, easily causes the oxygen 

 and the hemoglobin to unite, while the almost complete 

 absence of any oxygen pressure at the tissues, permits 

 their separation. The blood in its circulation constantly 

 flows from the place of high oxygen pressure at the lungs 



1 The oxygen pressure of the atmosphere is that portion of the total atmospheric 

 pressure which is due to the weight of the oxygen. Since oxygen comprises about 

 one fifth of the atmosphere, the pressure which it exerts is about one fifth of the 

 total atmospheric pressure, or, at the sea level, about three pounds to the 

 square inch (15 X \ = 3) . This is the oxygen pressure of the atmosphere. The low 

 oxygen pressure in the tissues is due to its scarcity, and this scarcity is due to its 

 entering into combination at the cells. 



