418 . BLOOD AND LYMPH. 



blood being a trifle more alkaline than the venous blood owing 

 to the fact that the latter contains more CO2. It is becoming 

 customary in physiological and medical literature to express the 

 hydrogen-ion concentration of the body liquids under normal and 

 pathological conditions in terms of the logarithms of the figures 

 giving the concentration, an expression which is known as the 

 hydrogen exponent, and is indicated by the symbol pn. These 

 logarithms are all negative, but the minus sign is omitted (see 

 p. 844). In this nomenclature the reaction of a neutral liquid is 

 expressed by the exponent 7. The reaction of blood for the 

 limits indicated above: 



Concentration 0.35 X 10~ 7 = 7.45 

 Concentration 0.49 X 10~ 7 = 7.31 



or the higher the exponent the greater the alkalinity of the blood. 

 Determinations of the reaction of the blood under normal and 

 abnormal conditions show that its range of variation in reaction 

 is very limited. Yet we know that in the metabolism of the 

 body acids and bases are being formed constantly and given to 

 the blood, and in our foods also we may ingest acids or alkalies, 

 which are absorbed into the blood and which, it might be sup- 

 posed, would change its reaction. Since the reaction remains 

 remarkably constant, it is evident that regulatory mechanisms 

 must be present to provide for the elimination or neutralization 

 of an excess of either acid or alkali. A general regulation of this 

 kind is provided in the activity of the lungs and the kidneys. 

 The concentration of CO 2 in the body is kept within certain normal 

 limits by the reflex regulation of the respiratory movements. 

 When the amount of C0 2 tends to rise, as in muscular exercise, for 

 example, the corresponding increase in the ventilation of the lungs 

 provides a method of eliminating the excess. So also the kidneys 

 furnish an abundant secretion by which any excess of acids or 

 bases is prevented from accumulating in the blood. In addition 

 to the regulatory methods of this kind it has been shown that the 

 composition of the blood itself is such that considerable amounts 

 of acids or alkalies may be added to it directly without altering 

 distinctly its reaction. This peculiarity is due to the presence in 

 the blood of salts of carbonic and phosphoric acid. We find in 

 the blood a mixture of monosodium phosphate and disodium 

 phosphate, sodium bicarbonate and carbonic acid, and it has 

 been shown, by Henderson especially, that such a mixture is 

 peculiarly adapted to preserve its neutrality within relatively 

 wide limits when acids or alkalies are added to it. A mixture of 

 this kind constitutes what is frequently called a buffer solution. 

 Monosodium phosphate is weakly acid, the disodium phosphate 

 is weakly alkaline, but a mixture of the two salts gives a neutral 

 solution, and addition of acid causes no change in the neutral 



