140 THE CIRCULATION OF THE BLOOD 



continuance of the saline injection, the blood pressure returns very 

 rapidly to its old level. The most striking result of such experiments is 

 the enormous volume of saline solution which can be slowly injected 

 without perceptibly affecting the pressure. The question is, Where does 

 the fluid go ? If the urinary outflow is examined, a certain increase will 

 usually be observed, but never by any means sufficient to account for 

 the disappearance of the injected saline. If we open the abdominal cav- 

 ity, we shall find that a considerable transudatioii of the saline into the 

 peritoneal cavity has occurred, and that the liver is conspicuously edem- 

 atous. A certain degree of edema is also usually evident in the tissues 

 of the extremities. 



Still more interesting and important, from a practical standpoint, are 

 the results obtained by injecting the above solutions into animals 

 whose blood pressure has been lowered by a previous hemorrhage. If 

 the blood removed during the hemorrhage is defibrinated (see page 101), 

 and then reinjected into the animal, it will bring the blood pressure al- 

 most but not quite back to its original level, which will then be fairly 

 w r ell maintained. If, on the other hand, saline solution instead of blood 

 is injected, the restoration of blood pressure (with an amount of saline 

 equal to that of the removed blood) will amount only to about three- 

 quarters of the extent to which it had fallen. This partial recovery is, 

 moreover, maintained for a short time only, after which the pressure 

 approaches the level to which it was reduced by the hemorrhage. 



These observations raise two important practical questions: (1) Why 

 is saline relatively ineffective in the restoration of pressure? and (2) 

 Why is t-he restored pressure not maintained? 



The answers to these questions brings us to a consideration of the next 

 of the factors concerned in the maintenance of the blood pressure, 

 namely, the viscosity of the blood. 



4, The Viscosity of the Blood 



The importance of this factor arises from the fact that facility of flow 

 in a tube is inversely proportional to the viscosity of the fluid and 

 directly proportional to the driving pressure to which it is subjected 

 that is, to the difference in pressure between two points in the tube. 

 If therefore the output of the heart remain constant, but the viscos- 

 ity of the blood be decreased by a saline injection, the facility of flow 

 will be increased and the pressure decreased. This fact can easily 

 be shown experimentally in a model by causing gum solutions of various 

 concentrations to be driven through a glass tube by means of a small 

 piston pump delivering a constant amount of fluid into the tube with 



