790 PHYSIOLOGY 



protein meal there is a considerable increase in the amount of amino-acids. 

 Thus the blood of fasting animals contains from 3-1 to 54 milligrams ammo- 

 acid nitrogen per 100 c.c. Blood taken after food contains 8-6 to 10-2 

 milligrams amino-acid nitrogen per 100 c.c. of blood. The question of the 

 fate of amino-acids thus absorbed from the intestine to the blood is decided 

 by an estimation of the amino-acid content of the different tissues after 

 the injection of amino-acids into the blood. Van Slyke has found that 

 after the injection of amino-acids only a certain proportion is excreted 

 with the urine, and that the rest of the amino-acids rapidly disappears 

 from the blood and is taken up by the tissues without undergoing any 

 immediate chemical change, though in the case of certain tissues, such 

 as the muscles, a definite saturation point exists which sets the limit to the 

 amount of amino-acids that can be absorbed. On the other hand, the 

 capacity of the internal organs, and especially of the li ver, for the absorption 

 of amino-acids is much greater. 



It is worthy of note however that the absorption of amino-acids by the 

 tissues from the blood is never complete, i. e. the amino-acids of the blood 

 must be in a state of equilibrium with those of the tissues, although the con- 

 centration in the latter may be much greater than in the former. If several 

 hours be allowed to elapse after the injection of amino-acids before the 

 analysis of the tissue is undertaken, it is found that the amino-acid nitrogen 

 content of the liver may have returned to normal, although the concentration 

 in the muscles has suffered no appreciable fall. Since we have evidence 

 that the circulation of amino-acids through the liver gives rise in this organ 

 to the formation of urea, we must conclude that this organ is especially 

 responsible for the breakdown of the products of protein digestion which are 

 not directly required for replacing tissue waste. This breakdown must 

 involve a process of deamination. We may therefore conclude that the 

 amino-acids normally produced by a protein digestion are absorbed without 

 further change into the blood stream. They then circulate throughout the 

 body, a certain proportion of them being built up in each tissue into the 

 proteins characteristic of that tissue in order to replace the waste caused by 

 wear and tear. The rest, probably the major part of the protein, is taken up 

 by the liver, where it undergoes deamination, the nitrogen moiety being 

 rapidly converted into urea and excreted by the kidneys, while the non- 

 nitrogenous moiety is carried to the working tissues to which it serves as a 

 ready and immediate source of energy. 



The fact that not only the blood but also the tissues contain amino-acids, 

 even after complete starvation for some days, shows that these substances 

 are intermediate steps not only in the synthesis but in the breaking down of' 

 body proteins. Free amino-acids are thus the protein currency of the body, 

 just as glucose is the carbohydrate currency. In the fasting body we must 

 regard the processes of autolysis as the main source of the amino-acids found 

 in the tissues, and it is by autolysis that the proteins of the resting tissues are 

 made available in starvation for those whose continued working is essential 

 for the maintenance of life. The fact that high protein feeding does not 



