416 BLOOD AND LYMPH. 



is, from 0.000000035 to 0.000000049 gram per liter. It is evident, 

 therefore, that the blood possesses a hydrogen-ion concentration 

 less than that of a neutral liquid, and is slightly alkaline, the arte- 

 rial blood being a trifle more alkahne than the venous blood owing 

 to the fact that the latter contains more CO2. Observations have 

 demonstrated that the reaction of the blood is kept remarkably 

 constant even under extreme conditions when much acid is added 

 to it from the products of metabohsm in the body. It is stated 

 that if the reaction fell to the neutral point, pH 7, or a little to the 

 acid side, pH 6.95, it would be fatal to the economy. This constancy 

 in reaction is due in the first place to the protective influence of 

 what are called buffer substances in the blood. These buffers are 

 the alkaline salts in the blood, namely, the alkaline carbonates, 

 especially sodium bicarbonate, HNaCOs; the alkahne phosphates, 

 dipotassium or disodium phosphate, K2HaP04; and the alkaline 

 salts of the proteins, especially of hemoglobin. This buffer effect 

 may be illustrated by the case of HNaCOs. This salt is present in 

 the plasma to the amount of about 0.25 per cent. If a fixed acid 

 such as lactic acid, which is formed during muscular contractions, 

 is added to the blood it does not increase the acidity to anything 

 like the extent that would happen if added to an equal amount of 

 water or to a solution of sodium chloride isotonic with the blood, 

 owing to the fact that it reacts with the sodium bicarbonate, form- 

 ing sodium lactate, a neutral salt, and liberating carbonic acid, 

 which is a very much weaker acid than the lactic acid. We have 

 in the blood, therefore, a large reservoir of alkali which takes care 

 of the CO2 and other acids formed in metabolism and prevents the 

 reaction from going too far to the acid side. It can be shown that 

 in such solutions the concentration of the hydrogen ions varies 

 with the ratio of the free acid, H2CO3, to the salt of that acid, 



for example, the HNaCOs, jj^^co ' ^^^ *^^* considerable vari- 

 ations in this ratio affect but slightly the acidity or concen- 

 tration of hydrogen ions. In the second place, any changes in 

 this ratio tend to be compensated by the regulatory action of the 

 lungs and kidneys. In muscular exercise in which much CO2 is 

 formed the ratio is raised shghtly and the reaction tends to become 

 more acid, but this slight increase in acidity stimulates the respira- 

 tory center, and the resulting increase in respiratory movements 

 acts to remove the excess of CO2 and thus restore the ratio to its 

 normal value. So also in the kidney we have an organ which ex- 

 cretes any excess of acid in the form of acid phosphate. We may 

 suppose that the reaction is essentially a conversion of alkaline 

 phosphate to acid phosphate, H2CO3 + HNa2P04 = HNaCOs + 

 H2NaP04, whereby the acid is excreted and the weak base HNaCOs 

 is retained in the blood. 



