418 BLOOD AND LYMPH. 



kaline reaction. In making the most accurate determinations of 

 the normal reaction of the blood it is necessary, therefore, to ex- 

 amine it at. the temperature of the body, and in an atmosphere con- 

 taining as much CO 2 as is present normally in the tissues of the body. 

 The results of the measurements of the reaction of the blood made 

 with these precautions indicate that it has a hydrogen-ion concen- 

 tration, varying from 0.35 X 10" 7 to 0.49 X 10" 7 , that is, from 

 0.000000035 to 0.000000049 gram per liter. It is evident, there- 

 fore, that the blood possesses a hydrogen-ion concentration less than 

 that of a neutral liquid, and is slightly alkaline, the arterial blood 

 being a trifle more alkaline than the venous blood owing to the 

 fact that the latter contains more CO 2 . 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 supposed, 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 regu- 

 lation 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 cer- 

 tain normal limits by the reflex regulation of the respiratory 

 movements. When the amount of CO 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 kidnej^s 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 di- 

 sodium 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 perceptible change in the 



* Henderson, "American Journal of Physiology," 21, 427, 1908; also 

 "Science," July 27, 1917. 



