434 VETERINARY PHYSIOLOGY 



innumerable small vessels, is subjected to greater and greater 

 friction — just as a river, in flowing from a deep narrow 

 channel on to a broad shallow bed, is subjected to greater 

 friction. 



Thus, in the arteries the powerful propulsive force of the 

 heart and the great resistance to outflow keep the pressure 

 high. 



When the capillaries are reached, much of the force of 

 the heart has been lost in dilating the elastic coats of 

 the arteries, and thus the inflow into the capillaries is much 

 weaker than the inflow into the arteries. At the same time, 

 the resistance to outflow is small, for, in passing from 

 capillaries to veins, the channel of the blood is becoming 

 less broken up and thus offers less friction to the flow of 

 the blood. 



When the veins are reached, the propelling force of the 

 heart is still further weakened, and hence the force of inflow 

 is very small. But there is no resistance to outflow from 

 the veins into the heart during diastole. Further, the great 

 veins, before they reach the heart, pass into the thorax, an 

 air-tight box in which, during each inspiration, a low 

 pressure is developed. 



What has been said of the pressure in the veins applies 

 equally to that in the lymphatics. 



2. Rhythmic Variations in Blood Pressure. 



Before considering the methods of investigating the 

 pressure in these different vessels, and the changes which 

 they undergo, certain rhythmic variations in pressure 

 may first be considered. 



A. Changes in Pressure Synchronous with the 



Heart Beats. 



1. The Arterial Pulse. 



With each ventricular systole, the contents of each 

 ventricle are thrown into the already full arteries, and the 

 pressure in these vessels is suddenly raised. 



