I JOS 



METABOLISM 



for example, the amino nitrogen of the liver has been observed to become 

 increased to 125 or 150 mg. per cent of the original amount. Although 

 this absorption of amino acids by the tissues is extremely rapid, it never 

 proceeds to such a point that the blood becomes entirely free of them. 

 Even after many days' starvation the blood contains its normal quota 

 of from 3 to 10 mg. per 100 gm. of moist tissue (Fig. 188). This indicates 

 that a certain equilibrium must become established between the amino-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. 



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Fig. 187. — Curves showing the amount of amino nitrogen taken up by different tissues after 

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

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



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 

 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 

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



