2 3 o TEXT-BOOK OF PHYSIOLOGY. 



7. Viscosity. Viscosity may be defined as the resistance to the move- 

 ment of the molecules of a fluid homogeneous body among themselves. In 

 accordance with the degree of this resistance, which may also be spoken of 

 as internal friction, will the fluid at a given temperature be mobile or viscous. 

 Viscosity varies partly with the nature of the fluid and partly on its tempera- 

 ture. Thus at the same temperature water, syrup, and pitch possess different 

 degrees of viscosity. A rise in temperature of i C. diminishes the viscosity 

 about 2 per cent. In all discussions relating to the viscosity of fluids, that 

 of distilled water is taken as a standard and regarded as unity. 



Blood as a fluid is regarded by physiologists as possessing viscosity, 

 though the definition in the foregoing paragraph is not strictly applicable, 

 as it is not a homogeneous but a heterogeneous fluid consisting of plasma 

 the molecules of which show an inner friction and of corpuscles which also 

 show friction. Blood having a complex composition as compared with 

 water has naturally a greater degree of viscosity or internal friction. Experi- 

 mental investigations render it certain that the observed viscosity is depend- 

 . ent on the corpuscular elements to a greater extent than on the composition 

 of the plasma. About two-thirds of the viscosity is due to the corpuscles. 



The viscosity of blood as compared with water may be determined by 

 .permitting the two fluids to flow through capillary tubes of corresponding 

 caliber under a steadily acting pressure and then determining the volume 

 that flows through each in a given time and comparing one with the other. 

 Normal human blood is thus found to possess a viscosity 4.5 times that of 

 distilled water at body temperature. Dog's blood has a viscosity 6 times 

 that of water. If the temperature of blood is raised the viscosity diminishes. 

 Recalling the statement that the viscosity is closely connected with the 

 presence of red corpuscles it would be expected that either an increase or 

 decrease in their number would change the viscosity in one direction or 

 another. In a case of polycythemia in which the red corpuscle count was 

 11,000,000 per cubic millimeter the viscosity was between 3 and 4 times the 

 normal. In certain other pathologic states of the blood characterized by a 

 diminution in the number of red corpuscles the viscosity diminished one-half 

 or more. The ingestion of meat raises the viscosity, while the ingestion of 

 fats and carbohydrates diminishes it. 



The determination of the viscosity for clinical purposes is accomplished 

 by the use of special forms of apparatus termed viscosimeters. These for 

 the most part consist of a capillary tube through which the blood is caused 

 to flow under the influence of a constant positive or negative pressure. The 

 distance the blood flows in a unit of time, compared with that of water, 

 represents the degree of viscosity. Among these apparatus those of Hess 

 and Determann are generally employed, descriptions of which will be found 

 in works on diagnosis. 



8. Coagulability. Within a few minutes after the blood is withdrawn 

 from the vessels of a living animal it begins to lose its fluidity, becomes some- 

 what viscid, and if left undisturbed passes rapidly into a semisolid or jelly- 

 like state. To this change in the physical condition of the blood the term 

 coagulation has been applied. The blood, during the progress of coagula- 

 tion, not only assumes the shape of the vessel in which it is contained, but 

 becomes so firmly adherent to its walls that it may be inverted without the 



