LECTURE XXV. 



THE ELIMINATION OF METABOLIC PRODUCTS FROM THE 



BODY. 



WE have already called attention to the important fact that the blood, 

 and especially the plasma or the serum, has a remarkably constant 

 composition. This holds true at least within certain limits, and in as 

 far as we can detect by means of our faulty methods of examination. 

 This relative constancy holds' not only, as we have shown, with regard 

 to the quantitative relations in which the substances are present, but 

 also, as far as we are able to see, for the qualitative relations. Thus we 

 feel justified in assuming, for example, that the amount of protein in the 

 blood may indeed vary somewhat, but it does not change its nature in 

 spite of the most varied forms of nourishment. 1 We intentionally speak 

 of a relative constancy, because there can be no such thing as an absolute 

 constancy in the composition of the blood, for from moment to moment, 

 according as this or that organ comes into action, the blood is bound to 

 receive quite different metabolic products. Such products are, however, 

 present in such a state of dilution that we can detect them only in a large 

 quantity of blood. Again, during the digestive process and when the 

 absorption and assimilation is at its height, sometimes this substance and 

 sometimes that one will circulate in the blood to an increased extent. 

 As a rule, however, these differences are so slight that they cannot be 

 detected by the present methods of analysis. They are usually concealed 

 by experimental errors. At the same time it is perfectly evident that 

 the composition of the blood is regulated to a remarkable extent and 

 kept constant within narrow limits. There are many ways in which 

 this regulation is effected, and they are not all of the same nature as is 

 usually assumed. First of all, the blood is kept from being flooded with 

 substances foreign to it, as they are contained in the food, by the activity 

 of the intestine. Were it not for this, we could not understand, as we 

 have repeatedly stated, why the composition of the serum should always 

 remain qualitatively and quantitatively practically the same. By means 

 of the activity of the digestive ferments, all of the complicated and 

 widely different nutrient substances, which vary from day to day, are 



1 Abderhalden and Samuely: Z. physiol. Chem. 46, 193 (1905). Emil Abderhalden: 

 Zentr. Stoffwechsel- und Verdauungskrankheiten 5, 647 (1904). 



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