170 Creatinine and Creatine in Blood 
corpuscles and plasma is unknown; but to this category belonged 
until quite recently creatinine and creatine. 
The distribution of materials within the blood is not only a 
matter of considerable interest in itself; it possesses an obvious 
bearing upon many questions of much wider import. In the 
transport of materials to and from the tissues every element of 
the blood may play a part; but their actual passage into or out of 
the circulation can be effected only through the plasma. Proc- 
esses like absorption, or excretion, or placental transmission 
may therefore raise problems for the solution of which a knowl-- 
edge of concentrations in the blood as a whole is insufficient, 
and the determination of plasma concentrations, which may be 
very different, becomes a prime necessity. Creatinine and crea- 
tine, in what is already known of their behavior under physio- 
logical and pathological conditions, present a number of problems 
of this description. 
For instance, although creatinine is being constantly excreted 
by the kidney, the laws which regulate its passage through that 
organ are unknown; any attempt to formulate them presupposes 
an acquaintance with the variations of the plasma creatinine, 
which may or may not parallel those of the creatinine of the entire 
blood. Creatine, on the other hand, in spite of the fact that its 
concentration in the blood is greater than that of creatinine, is 
usually absent from the urine of the male adult; but we do not 
know whether this is because it is localized exclusively in the 
corpuscles, or because its elimination, like that of glucose and 
chlorides, is dependent upon the crossing of a certain threshold of 
concentration in the plasma. The doubt could be resolved, and 
fresh light might be thrown upon the occasional occurrence of 
creatinuria, if we had precise information regarding the actual 
distribution of creatine in the blood. During pregnancy, to take 
a problem from another field, creatinine and creatine are found in 
the fetal as well as the maternal blood; and it is natural to in- 
quire to what extent and in what manner these substances share 
in the transfer of materials from one circulation to another across 
the placenta. Obviously the question demands for its answer a 
knowledge of their concentrations in the respective plasmas. 
Even in pathological conditions a study of the relations under 
discussion might well reveal something of interest. The accu- 
