METABOLISM 195 



similar fluid is kept circulating, from a study of the relationship 

 between oxygen consumed and carbon dioxide eliminated 

 (respiratory quotient), and from measurements of the heat pro- 

 duced by the body much valuable information has been obtained 

 about the processes going on in the tissues. 



Protein Metabolism. The metabolism of proteins, and 

 problems connected with this field make up one of the most im- 

 portant phases of metabolism. The reason for this is not difficult 

 to divine. Proteins make up the bulk of the solid material of 

 living tissue. Without an adequate supply of them in the diet, 

 an animal will die, even though it be given all it will eat of the 

 other food substances such at fats, carbohydrates, salts, etc. 

 Evidently proteins have some role to play which cannot be filled 

 by the other classes of foods. Since the other foods are good 

 fuels, and are easily burned in the body, the importance of the 

 proteins seems to be connected with furnishing building or repair 

 material to the tissue.s, or supplying certain chemical group- 

 ings required for the manufacture of some vitally important 

 products manufactured in the body itself. It will be of in- 

 terest to follow the proteins and the products formed from them 

 in their passage through the organism. 



In the digestive tract proteins are broken down by enzymes 

 into amino acids and in this form they are absorbed and pass 

 into the capillaries of the villi, and thence into the general cir- 

 culation by way of the portal vein. Until a few years ago it was 

 a matter of lively dispute whether the digestive products of the 

 proteins entered the blood in the form of amino acids, of pro- 

 teoses and peptones, or were rebuilt in the intestinal wall to 

 form proteins. It had never been possible to find amino acids 

 or proteoses and peptones in the blood, so that many investigators 

 believed these products were rebuilt into protein before passing 

 into the blood. One by one, pieces of evidence have accumulated 

 to settle this question, and we now know that normally the 

 greater part at least of the amino acids produced in digestion 

 gets into the blood as such, and is transported to the tissues. It 

 has been shown that if proteoses and peptones are injected into 



