904 PHYSIOLOGY 



of the part from which the sample is taken. It is also increased as a 

 result of any operation upon a serous cavity in consequence of exuda- 

 tion of plasma in the inflamed or irritated part. It is diminished as the 

 result of bleeding. The specific gravity of serum is 1028 to 1032, of cor- 

 puscles about 1090. It is interesting to note that the specific gravity of 

 the blood is highest in the foetus at full term, when it amounts to 1066, 

 contrasting with that of the mother at the same time, the specific gravity 

 of whose blood is only 1050. The specific gravity rapidly falls to the 

 latter figure after birth. 



THE REACTION OF THE BLOOD 



The blood has long been described as alkaline owing to the fact that it 

 turns neutral litmus paper blue. This fact can be demonstrated by allowing 

 a drop to flow on a piece of glazed litmus paper and then wiping away the 

 blood with a piece of linen moistened with distilled water or neutral saline 

 solution. The alkalinity of the blood was determined by mixing a small 

 definite quantity with sulphate of soda solution containing a definite amount 

 of tartaric acid. The acid was then titrated against a decinormal solution 

 of sodium hydrate until a drop of the mixture gave a blue stain and was 

 placed on blue litmus paper. It must be noted however that this method 

 gave, not the alkalinity, but a measure of the alkaline reserve i. e. of the 

 total amount of soda in combination with weak acids which can be replaced 

 by the tartaric acid. This alkaline reserve consists almost exclusively of 

 sodium bicarbonate, and the method indicated above is a means of estimating 

 the total amount of this salt in the blood. 



In van Slyke's method the alkaline reserve of the plasma is determined by finding 

 out how much CO 2 is evolved from a given volume of the oxalated plasma (1 G.C.), when 

 this is treated with 5 per cent, sulphuric acid so as to convert all the bicarbonate into 

 sulphate. Since the amount of carbonic acid taken up depends on the partial pressure 

 of the C0 2 in the atmosphere to which the plasma is exposed, the plasma is first shaken 

 up with alveolar air provided by the experimenter himself, which always contains about 

 55 per cent, of C0 2 . 



Normal human blood plasma treated in this way yields between 0-6 and 0*7 c.c. of 

 C0 2 per cubic centimetre. 



The reaction of the fluid, strictly speaking, depends on the. relative pro- 

 portions of the H and OH ions present. Pure distilled water owes its 

 neutrality to the fact that it contains equal amounts of H and OH ions. 

 If the H ions increase and the OH ions diminish, the reaction becomes acid. 

 The relative concentration of H and OH ions in a fluid can be measured 

 electrically. In this method the potential difference is measured between 

 the fluid and a platinum electrode immersed in it, which is kept saturated 

 with hydrogen. In determining the reaction of the blood by this moans. 

 care must be taken to make the estimation at the body temperature, and 

 also to keep a tension of 5 per cent, of an atmosphere of C0 2 in the gaseous 

 mixture in contact with the blood or blood plasma. If blood is raised from 



