THE BLOOD. 55 



one-fifth of oxygen, and as the combined pressure of these 

 two gases is, generally speaking, fifteen pounds to the 

 square inch, it is evident that the oxygen part of that pres- 

 sure is one-fifth of that, or three pounds, while in an atmos- 

 phere diminished to one-sixth of fifteen pounds, or two and 

 one-half pounds, the oxygen part of this would be one-sixth 

 of three pounds, or one-half pound. 



To state it again, haemoglobin will unite with oxygen 

 when it is exposed to an oxygen pressure of one-half pound 

 or more, and will not only refuse to unite, but actually dis- 

 unite when it is exposed to an oxygen pressure less than 

 one-half pound. As the oxygen pressure in the lung is 

 three pounds, it explains the union with that gas there, 

 while the fact that it gives up its oxygen in the tissues is 

 accounted for by the simple physical reason that the oxygen 

 pressure in the tissues is less than one-half pound, as the 

 oxygen there is being continually used up by the hungry 

 tissues. Why this haemoglobin should be " boxed up " in 

 corpuscles and not simply dissolved in the plasma of the 

 blood, is evident. It could carry as much oxygen in one 

 case as in the other; but as much of the plasma soaks 

 through the capillaries, becomes lymph, bathes the tissues, 

 and only after a considerable time is finally poured back 

 into the blood stream, it means that any haemoglobin dis- 

 solved in this would have been able to carry but one load, 

 instead of a hundred, possibly, by being whirled along with 

 the blood stream. 



When combined with oxygen (then called oxyhaemo- 

 globin) it is of a bright red color, the color of arterial blood. 

 Haemoglobin is an albuminous compound characterized by 

 a relatively large amount of iron, although in actual quan- 

 tity the iron contained is less than one-half of one per cent. 

 This iron, however, plays a very important role in the 

 oxygen-carrying process. The rapidity with which ordi 

 nary iron unites with oxygen, or as we say, " rusts," may 

 account for its presence here. 



