644 



METABOLISM 





amino acids than during its passage through the entire remainder oi' 

 the body. 



It will be seen that the above conclusions are drawn from estimations 

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

 artery, the upper curve 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 



Fig. 189. Curves showing the concentration of amino-acid nitrogen in the blood during fasting 

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



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 

 the two experiments taken together, indicate that the amino acids after 

 their absorption can not remain in the tissues, in a free condition for a 

 long time. It means that the amino acids during natural digestion must 

 be disposed of at a rate which is practically the same as that at which ab- 

 sorption is proceeding. 



