RESPIRATION BEYOND THE LUNGS 379 



THE TRANSPORTATION OF GASES BY THE BLOOD 



The Transportation of Oxygen 



It is plainly not by mere solution in the plasma of the blood that the 

 transportation of 2 occurs, for at the partial pressure of this gas ex- 

 isting in the alveolar air at the temperature of the body the amount that 

 could be dissolved in the blood would be only one-fortieth of that which 

 is actually found to be present. If there were only plasma in the blood 

 vessels, it would require a volume of fluid amounting to 150 kilograms 

 or more in order to convey the necessary amount of 2 from the lungs 

 to the tissues; that is, the contents of the vascular system would weigh 

 twice as much as the average Aveight of a man. 



The substance that carries the 2 in the blood is the hemoglobin, which 

 may be described as a highly complex iron compound of protein espe- 

 cially evolved for the purpose of transporting 2 . In some of the lower 

 animals other compounds exist in the blood for this purpose, but none 

 of them is to be compared in its efficiency with hemoglobin. They are 

 merely poor imitations of it. 



Regarding the conditions under which hemoglobin combines with or 

 delivers up 2 , the first question that presents itself is whether or not 

 the reaction is a strictly chemical one. If so, a definite amount of 0, 

 must be capable of combining with a definite amount of hemoglobin. It 

 is impossible to secure hemoglobin of sufficient purity to test this rela- 

 tionship directly on hemoglobin itself, so that we must test it indirectly 

 by examining the combining equivalent between 2 and that portion of 

 the hemoglobin molecule upon which the combining power depends. This 

 is the part of the molecule containing iron. Now, if we compare the 

 amount of 2 which hemoglobin can take up with the amount of iron 

 present in the hemoglobin, we shall find that one atom of iron becomes 

 combined with two atoms of 2 . Evidently, then, we are here dealing 

 with a definite chemical reaction occurring between the 2 and the iron 

 of the hematin portion of the hemoglobin. This relationship is known 

 as "the specific oxygen capacity of hemoglobin. " 



In showing that the union of 2 and hemoglobin occurs according to 

 chemical laws, we throw into prominence consideration of the mechanism 

 by which the 2 combined with hemoglobin in the blood is rapidly de- 

 livered up in the capillaries so as to supply the tissues with their require- 

 ment, and is then as rapidly recombined again in the lungs. Moreover, 

 we must reconcile facts implied by the idea of a specific 2 capacity with 

 the well-known observation that the hemoglobin in the circulation is 

 usually united with considerably less 2 than the total amount possible. 



