642 



METABOLISM 



In the Tissues. After entering the circulation, the amino acids very 

 quickly disappear from it again. This has been demonstrated by ob- 

 serving the amount of amino acids in the blood after intravenously 

 injecting a solution of amino acid into an anesthetized animal. After 

 injecting 12 gm. of alanine into the vein of a dog, 90 per cent was found 

 to have disappeared from the circulation within five minutes. The ques- 

 tion is, What becomes of the amino acids that rapidly disappear? Are 

 they decomposed in the blood, or do they become absorbed by the tis- 

 sues? This problem has been attacked by analyzing portions of various 

 organs and tissues removed before and some time after the injection 



Fig. 187. Vividiffusion apparatus of J. J. Abel. 



into an animal of solutions of amino acids. In the case of the muscles it 

 has been found that the amino-acid content increases until from 60 to 

 80 mg. per cent of amino acid has accumulated. Beyond this point, 

 however, the muscles do not seem to be able to take up any more amino 

 acid (Fig. 188). The capacity of the abdominal organs, however, is more 

 elastic; 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. Al- 

 though 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, 

 for even after many days' starvation the blood contains its normal quota 

 of from 3 to 10 mg. per cent (Fig. 189). This indicates that under these 



