18 BLOOD 



ence between a strong and a weak acid is due to the greater 

 degree of ionisation of the former. 



It is usual to express the hydrogen ion concentration 

 as the logarithm to base 10 of the hydrogen ion concentra- 

 tion, according to Sbrensen's method, the negative sign 

 being omitted for simphfication. This figure is known as 

 the " Ph." 



When the H-ion concentration is 1 x 10"''' normal, Ph = 

 7-0. When it is 0-2 x 10- ^ normal, Ph = 7-7. 



(Since log 10*2 = 0-30 

 .-. -2 X 10-7= 10-3o-7o^_7.7)_ 



In the case of blood, Ph • 7-0 and Ph 7-7 are the hmits 

 compatible with health. The figure for Ph decreases as 

 the H-ion concentration (and therefore the acidity) increases. 



When an acid is added to the blood the H-ion concentra- 

 tion is not raised to anything like the same amount as 

 occurs when the acid is added to water. The stabihty 

 of the blood in this respect is called buffer action. Buffer 

 action may therefore be defined as the capacity to take 

 up acid without acquiring a corresponding acidity. The 

 substances responsible for buffer action, themselves known 

 as buffers, are chiefly inorganic salts, and to a less extent 

 proteins. Of the salts the most important is NaHCOg, 

 which for practical purposes may be considered to be the 

 only " buffer" normally called into play. When an acid 

 such as lactic is added to blood the following reaction 

 occurs — 



NaHCOg -f Hi7= NaL-f H2CO3. 



Since carbonic acid is hardly ionised at all, there is practi- 

 cally no change in Ph. 



A solution of NaHCOg always contains a certain amount 



of CO2 dissolved in it, and the Ph of such a solution is 



TT r\r\ 



determined by the ratio ■vpTrT^- When, therefore, lactic 

 acid is added to circulating blood, the diminution in the 



