112 NUTRITIONAL PHYSIOLOGY 



free feeding of carbohydrates, nor does it noticeably di- 

 minish during long fasting. The explanation of this sin- 

 gular constancy will be given later. 



Fat, or its derivatives, is found in the plasma in a pro- 

 portion of a similar order to that of sugar. It is much more 

 subject to variation, rising notably after a meal in which 

 there was much fat. Plasma obtained from a dog at such 

 a time exhibit the phenomenon of developing a true cream, 

 the fat gathering at the surface. During starvation the 

 blood is not necessarily poor in fat, since it is likely to 

 be engaged in carrying this form of food from places of 

 storage the adipose tissue to the muscles and elsewhere 

 to be oxidized. 



Those compounds in the plasma which we can confi- 

 dently designate as waste-products occur only in the small- 

 est quantities. Carbon dioxid is, of course, an exception, 

 being very abundant. Among the non-gaseous wastes 

 destined to be dealt with by the kidneys, urea is the only 

 one which is easily detected. This is the compound in 

 which seven-eighths of the nitrogen is carried from the 

 body.. The fact that the waste substances are kept down 

 to such a low level in the blood is evidence of the remark- 

 able efficiency of the kidneys and the supplementary or- 

 gans of excretion. It is also a reminder of how rapid and 

 copious is the circulation. No portion of the blood can 

 long escape passage through the glands which have this 

 striking power to hold it to a standard composition. 



If the mixed solids from a sample of plasma are inciner- 

 ated, we have left a small mass of ash or mineral matter 

 amounting to about 1 per cent, of the whole blood. By 

 far the largest component is sodium chlorid, the chief salt 

 of the diet, and the only one which we deliberately add to 

 our food. Other bases represented are potassium, calcium, 

 and magnesium. Besides chlorids, we find carbonates and 

 phosphates, the former having greatly involved relations 

 with the carbon dioxid of the blood. To the list which has 

 been given might be added many minor constituents, some 

 of which are judged to be present because of certain prop- 



