144 INTRODUCTION TO GENERAL PHYSIOLOGY 



the reduced bicarbonate, thus bringing back the ratio of carbon 

 dioxide to sodium bicarbonate to its normal value. A converse 

 process accommodates to an increased alkalinity, although it 

 appears to be less effective, perhaps because the requisite decreased 

 ventilation implies a diminution in the supply of oxygen. 



In addition to this method there are two other means of main- 

 taining the neutrality of the blood. In the first place, an increase 

 in the acidity of the blood causes, in some way, a retardation of 

 the formation of urea from the ammonia resulting from the 

 deamination of amino-acids in the liver (p. 77). This ammonia 

 then neutralises acid, and the salt formed is excreted in the urine. 

 It has recently been found that if excess of alkali is introduced 

 into the blood, lactic acid is formed in the organism. This com- 

 bines with the base present in excess, and the neutral lactate 

 appears in the urine. In the second place, the kidney itself acts 

 as a regulator. As the reader is doubtless aware, there are two 

 phosphates of sodium or potassium ; one of these (NaH 2 PO 4 ) is 

 acid, the other (Na 2 HPO 4 ) is alkaline. A mixture of the two in 

 certain proportions is neutral. They are contained in small 

 amount in the blood in this latter ratio. If the blood becomes 

 j more acid, a larger proportion of the acid phosphate is formed. 

 This passes into the glomerular filtrate. The phosphates in solu- 

 tion are hydrolytically dissociated, so that there are present sodium 

 hydroxide and phosphoric acid. Sodium salts are wanted by the 

 body, while phosphoric acid is only wanted in very small amount. 

 The tubules, therefore, absorb the sodium in the form of bicarbonate, 

 leaving the phosphoric acid in slight excess, so that the urine is 

 usually slightly acid. It will be clear that if the phosphoric acid is 

 in greater excess than normal in the glomerular filtrate, it will be 

 left behind and escape with the urine, while a fluid of normal re- 

 action will be absorbed into the blood. 



Viscosity. The molecules of a liquid experience friction in 

 moving over one another, so that if a part of the liquid is at rest 

 while another part of it is in motion, there is friction between the 

 two. This internal friction is the cause of that property known as 

 viscosity, familiar to all in the difference between water and glycerin 

 or treacle. If there are particles in the liquid, such as the blood 

 corpuscles, the viscosity is greatly increased, partly owing to the 

 fact that where a liquid is in contact with a solid, a film of it is 

 held stationary, so that there is more friction in the whole mass 

 than if the corpuscles were absent. Blood corpuscles are also 

 deformed in shape when passing through narrow channels, and this 

 serves to increase the apparent viscosity, since a part of the energy 

 of the current is taken up in the changing of shape. Indeed, the 

 viscosity of the plasma is only about half as much again as that of 



