GENERAL PROPERTIES: THE CORPUSCLES. . 417 



HCO 3 and OH. In accordance with these facts, it is evident that 

 to determine whether the blood is neutral, acid, or alkaline it is 

 necessary to ascertain its concentration in hydrogen and hydroxyl 

 ions. As a matter of fact, it is sufficient to determine the con- 

 centration in either hydrogen or hydroxyl ions since the product 

 of the two forms a constant: 



H X OH = Constant. 



For technical reasons it has proved to be more convenient to 

 determine the concentration in hydrogen ions, and the reaction 

 of the blood is usually expressed in terms of the hydrogen-ion 

 concentration. Two methods are employed in making these 

 determinations.* One, the electrical method, makes use of 

 platinum electrodes covered with hydrogen gas, and measures the 

 difference in electric potential between this electrode and the 

 blood, and the other is the method of selected indicators. A 

 series t>f indicators may be selected which undergo a change in 

 color at certain concentrations of hydrogen ions, and under suit- 

 able precautions the color change given to these indicators by the 

 blood may be interpreted in terms of the hydrogen-ion concentra- 

 tion. In the first determinations of the reaction of the blood made 

 by the electrical method slightly erroneous figures were obtained, 

 owing to the fact that precautions were not taken to examine the 

 blood under its normal carbon-dioxid tension. In the body the 

 blood is surrounded by a medium containing carbon dioxid under 

 a certain pressure, for venous blood about 5 per cent, of an atmo- 

 sphere, or 40 mm. The carbon dioxid gives the blood an acid 

 reaction, hence if the blood is taken out of the body and exposed 

 to an atmosphere practically free from CO 2 , it will tend to give off 

 some of this gas and take on a slightly more alkaline reaction. 

 In making the most accurate determinations of the normal reac- 

 tion of the blood it is necessary, therefore, to examine it at the 

 temperature of the body, and in an atmosphere containing 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 are as follows: In a neutral liquid, pure water for 

 example, the hydrogen-ion concentration is equal to 1 X 10~ 7 

 (normal), or 0.0000001 gm. hydrogen per liter. Blood under the 

 normal conditions in the body has a hydrogen-ion concentra- 

 tion, varying from 0.35 X 10~ 7 to 0.49 X 10~ 7 , that is, from 

 0.000000035 to 0.000000049. It is evident, therefore, that the 

 blood possesses a hydrogen-ion concentration less than that of a 

 neutral liquid, and it is therefore slightly alkaline, the arterial 



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

 "Science," March 14, 1913. Consult also Hasselbach, "Biochemische Zeit- 

 schrift," 30, 317, 1911, and Michaelis and Davidoff, ibid., 46, 131, 1912. 

 27 



