498 THE HUMAN BODY 



Glycosuria from Increased Permeability of the Kidney-Cells to 

 Sugar. The injection of a certain drug, phlorhizin, into the cir- 

 culation is followed by a glycosuria which is due chiefly, although 

 probably not wholly, to alterations in the kidney. These are of 

 such a sort that the kidney-cells, instead of removing only sugar 

 in excess of 0.2 per cent, take all that comes to them. The result, 

 of course, is a great waste of this valuable fuel, requiring greatly 

 increased consumption of carbohydrates to make it good. This 

 form of glycosuria has been produced experimentally in animals, 

 for purposes of study, but occurs rarely, if at all, as a disease of 

 man. 



The Absorption of Proteins. We have learned that the digest- 

 ive process splits proteins into their constituent amino acids 

 (p. 466). The advantage of this is obvious, when we recall the 

 fact that an important function of proteins is to repair tissue waste, 

 and the further fact that to do this the food protein must be con- 

 verted into the characteristic tissue protein of which it becomes a 

 part. We saw in Chap. I (p. 11), that the difference between 

 one protein and another is in the number, proportions, or arrange- 

 ment of the amino acids which make up their molecules. While 

 the food proteins, as such, would not serve for tissue repair, the 

 amino acids of which they are composed are precisely what the 

 Body needs for rebuilding its own substance. Furthermore, the 

 different tissues must differ somewhat in the constitution of their 

 characteristic proteins, and for the repair of all the different 

 tissues a mixture of amino acids is evidently much more 

 useful than a small number of undigested food proteins could 

 possibly be. 



There is abundant evidence that the digested amino acids are 

 absorbed directly into the blood-stream, and not into the lacteals. 

 This has been proven by inserting a tube into the thoracic duct of 

 an animal and draining off all the lymph produced during the 

 absorption of a protein-rich meal. No increase in the percentage 

 of nitrogen (the characteristic element of proteins) could be de- 

 tected in the lymph; conclusive proof that the amino acids do not 

 follow that pathway. Moreover, chemical methods recently de- 

 vised have proven the presence of amino acids in the blood-stream, 

 and that during the absorption of a meal of meat, they are in- 

 creased in amount. The use the Body makes of these amino acids 



