574 METABOLISM, NUTRITION AND DIETETICS 



occurs in the wall of the alimentary canal than is necessary for the 

 needs of the tissues composing it, and, perhaps in addition, for the 

 maintenance of the normal composition of the plasma, and that the 

 decomposition products of the proteins are mainly absorbed as such, 

 and pass in the blood to the tissues for which they are destined. If 

 this is the case, the blood-proteins can no longer be looked upon as 

 representing the main current of protein supply for the organs, but 

 rather the store of protein material proper to the circulating tissue 

 blood itself, and which confers on it certain chemical and physio- 

 chemical properties (e.g., the due degree of viscosity) necessary for 

 its function. Slowly accumulated, under ordinary conditions, and 

 slowly consumed, this protein store may, of course, be at the dis- 

 posal of the organs in an emergency for instance, in starvation 

 or may be rapidly recruited from the organ-proteins, as after 

 haemorrhage, just as in prolonged hunger the proteins of skeletal 

 muscle may be utilized to feed the heart. That the blood-proteins 

 can serve as nutritive material for the cells without undergoing 

 digestion in the alimentary canal is well shown by the observations 

 of Carrell and Burrows on the growth of isolated tissues in a medium 

 composed of clotted blood-plasma. But, as previously pointed out 

 in another connection (p. 449), a portion, and probably a large 

 portion, of the digested protein is absorbed from the intestine by 

 the blood in the form of amino-acids. Considerable quantities of 

 these compounds can be separated by dialysis from blood drawn 

 off during the absorption of proteins or by the process of vivi- 

 diffusion (p. 48) (Abel). Among these amino-acids, glycocoll, 

 alanin, glutaminic acid, and leucin, have been identified. While 

 the quantity of amino-acids in the blood, which is very small in the 

 fasting animal, is decidedly increased during protein digestion, it 

 is probable that even in starvation amino-acids derived from the 

 decomposition of the body-proteins are not entirely lacking. The 

 normal concentration of amino-acids in the general blood of man and 

 of the dog is about o-i per cent. i.e., about the same as that of 

 dextrose, and it may be nearly twice as great in the portal blood 

 of dogs after a heavy protein meal. The amino-acids are very 

 rapidly absorbed by the tissues from the blood, and can be demon- 

 strated in muscles and other organs as free amino-acids. They 

 accumulate there in much greater concentration than that in which 

 they exist in the blood. Accordingly, the tissues take them up from 

 the blood by some other process than simple diffusion (Van Slyke) . 

 It has been surmised that amino-acids constitute the form in which 

 proteins are transported from tissue to tissue, as well as the form in 

 which proteins are normally utilized by the cells.* Although this 

 cannot be regarded as yet established, there is reason to believe that 



Recent experiments of Abel tend to rehabilitate the old view that some 

 protein is absorbed as proteoses, which can also be isolated from the tissues- 



