26 THE CIRCULATING LIQUIDS OF THE BODY 



reaction, and a considerable amount of acid must be added before the 

 colour of the indicator which denotes acidity appears. The true re- 

 action of the serum is not, of course, at one and the same time both 

 alkaline and acid ; but it is so near neutrality that it falls just below the 

 degree of alkalinity necessary to give the pink colour with phenol- 

 phthalein, and just below the degree of acidity which gives the pink 

 colour corresponding to an acid reaction witlmiethyl orange. Certain 

 indicators for example, rosolic acid turn so as to give sharp colour 

 reactions at about the concentration of hydrogen and hydroxyl ions 

 in the blood, and these may possibly be of use in determining the 

 changes in the true reaction for clinical purposes (Adler) . 



More closely related to the true alkalinity of the blood than the 

 titratable alkalinity is the carbon dioxide content. The estimation 

 of the total carbon dioxide in a sample of blood throws light upon 

 the capacity of the blood to perform one of its most important 

 functions the transportation of carbon dioxide and to preserve 

 one of its essential properties an almost neutral reaction in the 

 presence of an excessive intake or production of acid substances. 

 In herbivorous animals the carbon dioxide content of the blood is 

 easily lessened by the administration of acids, but in carnivora 

 and in man it is much more difficult to bring about such a decided 

 effect, for the reason already mentioned, the acid being neutralized 

 by ammonia. In many diseases, however, and particularly in those 

 accompanied by fever, this protective mechanism breaks down. 



Specific Gravity of Blood. The average specific gravity of blood 

 is about 1066 at birth. It falls during infancy to about 1050 in 

 the. third year, then rises till puberty is reached to about 1058 in 

 males (at the seventeenth year), and 1055 in females (at the four- 

 teenth year). It remains at this level during middle life in males, 

 but falls somewhat in females. In chlorotic anaemia of young 

 women it may be as low as 1030 or 1035. It rises in starvation. 

 Sleep and regular exercise increase it (Lloyd Jones).* The specific 

 gravity of the serum or plasma varies from 1026 to 1032. 



The Electrical Conductivity of Blood. The liquid portion of the 

 blood conducts the current entirely by means of the electrolytes 

 dissolved in it, the most important of these being the inorganic 

 salts; and the conductivity of the serum varies, in different speci- 

 mens of blood, within a comparatively narrow range. The con- 

 ductivity of entire (defibrinated) blood, on the contrary, varies 

 within wide limits. For instance, in a case of pernicious anaemia 

 the conductivity of the blood was found to be almost double that 

 of normal human blood, while the conductivity of the serum was 

 normal. The most influential factor which governs this variation 



* In 165 students (male) the average specific gravity of the blood, as deter- 

 mined by Hammerschlag's method (p. 62) was 1054*4. In 149 of these the 

 variation was from 1050 to 1065; in 94 (or 57 per cent, of the whole), from 

 1054 to 1060; in 4, from 1046 to 1049; in 9, from 1066 to 1070. In 3 the 

 specific gravity was only 1040 to 1042. 



