THE METABOLISM OF PROTEIN 



609 



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

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

 parts while absorption of protein from the intestine is in process, it 

 has been found, as shown in Fig. 188, that during 1 the passage of the 

 blood through the liver there is a greater fall in the concentration of 

 amino acids than during its passage through the entire remainder of 

 the body. 



It will be seen that the above conclusions are drawn from estima- 

 tions made on blood taken from the vena cava, portal vein, and hepatic 



O 



Fig. 188. Curves showing the concentration of amino-acid nitrogen in the blood during fasting 

 and protein digestion. (From D. D. Van Slyke.) 



artery, the upper curves in the chart being from animals during digestion 

 and the lower from fasting animals. The results show that the liver must 

 be particularly greedy of amino acids, which, however, must rapidly be- 

 come transformed into other substances, since no conspicuous varia- 

 tion has been found to occur in the amino-acid content of the tissues 

 according to whether the animal is fasting or is digesting protein food. 

 This result, it is to be noted, is quite different from that which is ob- 

 tained after the intravenous injection of amino acids, and the results of 



