242 PRINCIPLES OF GENERAL PHYSIOLOGY 



artery merely a small fraction of it was doing so. The sectional arcu of the 

 arterioles taken together may clearly be even greater than that of the artery, 

 without affecting the nature of the result, although the effect will be less, on 

 account of the less rate of flow. 



It is important to bear in mind that the peripheral resistance of the arterial 

 system, resulting from the division into small arterioles, is due entirely to the 

 internal friction of the blood, not to friction against the walls of the vessels ; 

 except indirectly, in so far as it is this latter friction which determines the 

 stationary condition of the blood film in contact with them. 



Frictional resistance in fluids being proportional to the square of the rate at 

 which the rubbing surfaces glide over one another, we see why there is comparatively 

 little resistance in the capillaries. Owing to the enormous increase of total sectional 

 area, the rate of flow is far less than in the arterioles. 



Now, the total amount of the friction experienced by the blood obviously 

 depends on that property of liquids known as internal friction, which differs greatly 

 in different cases ; compare water with treacle, for example. It is, therefore, of 

 some importance to find out what are the various conditions on which this property 

 depends. 



We have to consider homogeneous liquids, such as pure liquids and true solutions, 

 colloidal solutions and suspensions, such as that of blood corpuscles in plasma. 



Chemical Composition. As a rule, the internal friction increases with the 

 molecular weight and, in homologous series, in proportion thereto. The increase 

 of the viscosity of water, due to the formation of polymers of a higher molecular 

 weight, has been discussed above. 



Temperature. Rise of temperature causes considerable decrease of viscosity, 

 as is known to every one in the case of such liquids as castor oil, glycerol, etc. 

 Hence also the necessity of using a thick lubricating oil for the cylinder of an air- 

 cooled petrol motor ; the high temperature would make another one too thin to serve 

 its purpose. The viscosity of blood diminishes to a large extent as the temperature 

 is raised, so that less work is demanded of the heart in order to drive a given amount 

 of blood through the arterioles ; or the same work will drive the blood at a greater 

 rate. This is an incidental advantage possessed by warm-blooded animals. 



Blood. Changes in the viscosity of blood, other than those produced by 

 differences of temperature, are also of importance. The presence of corpuscles 

 increases the viscosity, which is therefore lower in defibrinated or " laked " blood 

 than in normal blood. Dilution has also the effect of diminishing viscosity, so 

 that a dilute blood passes more rapidly through the renal vessels and the excretion 

 of urine is favoured. 



Viscosity of Colloidal Solutions. The internal friction of the blood plasma, 



as a colloidal solution, is affected by the same factors as those which act on 



that of colloidal solutions in general. A brief account only can be given 



here ; the reader will find more details in the report of the discussion at the 



Faraday Society on 13th March 1913. As regards suspensoids, the degree 



of dispersity is the main factor, and it appears that the maximum of viscosity 



is at medium values of dispersion, being less with very small as well as with 



very large particles. It is uncertain whether this is connected with the variable 



amount of the dispersion medium associated with the particles. Emulsoids show 



great variety of changes in viscosity, so that the determination of this property 



is a valuable one in the investigation of such systems (see the paper by Wo. 



Ostwald, 1913, from which the following statements are chiefly derived). I 



have already referred to the effects of concentration, temperature and degree 



of dispersion. Other factors are solvate formation ; electrolytic dissociation, in 



which solvate formation is probably involved ; previous thermal treatment, as 



in the case of gelatine, which also shows an influence of mechanical treatment, 



even in the liquid state, in that its viscosity diminishes by repeated passage 



through a narrow tube and gives evidence of some kind of " structure " ; inoculation 



with Mnall quantities of a more viscous colloid, which produces a much greater 



effect than that due to its own viscosity ; time, especially shown by the effect of 



the rate at which the temperature is changed ; and finally the addition of 



