THE BLOOD 111 



proportion is little increased by drinking and little reduced 

 by thirst. Its constancy is secured by kidney activity 

 coupled with rapid exchanges of fluid which take place be- 

 tween the blood and the lymph and tissues. When a great 

 draft is made upon the water of the blood, as is the case in 

 profuse sweating, the volume is restored by taking in water 

 from outside the vessels, and this may mean a marked 

 loss of body weight. The blood itself is not likely to share 

 appreciably in such a loss. Its total volume in the adult 

 of average build has been variously estimated at from 3 to 

 5 quarts. The earlier calculations led to the higher figures; 

 4 quarts (8 pounds or -^ of the weight) is a reasonable 

 standard for us to adopt. 



Among the substances in the plasma which can be classed 

 under the general head of foods, proteins take the first 

 place. They form about four-fifths of the total solids. 

 This high proportion does not correspond at all with the 

 make-up of an average diet, in which, as we have seen, 

 proteins take the second or even the third place. It is 

 usual to distinguish three varieties of protein in the plasma, 

 serum-albumin, serum-globulin, and fibrinogen, but indica- 

 tions are multiplying which support the belief that the ac- 

 tual number of proteins with clearly individual characters 

 is much greater. Our knowledge of these bodies, their 

 mode of origin, place of formation, and the particular 

 value of each one in the system, is in a highly unsatisfac- 

 tory state. 



Carbohydrates, which occupy the leading position in the 

 rations ordinarily chosen by civilized man, are very scantily 

 represented in the plasma. The principal one present is 

 the monosaccharid, dextrose, known also as glucose, or 

 rape-sugar. The percentage of dextrose figured for the 

 rhole blood is between 0.1 and 0.2. This means from 1 

 2 grams of sugar in a liter of blood, and limits the total 

 imount in circulation to 10 grams at most. Such a quan- 

 tity seems insignificant when it is remembered that 100 

 rams of sugar may easily be formed in the digestion of a 

 loderate meal. The sugar is not readily increased by 



