THE METABOLISM OF PROTEIN 



643 



conditions a certain equilibrium must become established between the 

 ammo-acid content of the blood and that of the tissues, the concentration 

 in the tissues being approximately from five to ten times greater than in 

 the blood. 



The absorbed amino acids are very loosely combined with the tissues, 

 for they can be extracted by such feeble reagents as water or dilute al- 

 cohol. Their presence can not, however, be merely due to diffusion; 

 for if it were, the concentration could not become greater in the tis- 

 sues than in the blood. The further fate of the amino acids is difficult 



150 



100 



50 



Injectio 



Muscle 



NH 



1 2 



Hours 



Fig. 188. Curves showing the amount of amino nitrogen taken up by different tissues after 

 the intravenous injection of amino acids. The lowermost curve shows the urea concentration of the 

 blood. (From D. D. Van Slyke.) 



to follow. We know that they do not remain in the body for a long time, 

 because most of the protein nitrogen in the food is excreted as urea 

 within twenty-four hours after ingestion; and when single amino acids 



I are fed, they quickly reappear in the urine as urea. 

 The tissues can therefore be only a stopping-place for the amino 

 icids. When the latter are determined in blood collected from different 

 3arts while absorption of protein from the intestine is in progress, it 

 las been found, as shown in Fig. 189, that during the passage of the 

 )lood through the liver there is a greater fall in the concentration of 



