CIRCULATION 261 



whole contractile force of each ventricle into the corresponding 

 artery ; and because the resistance offered to the outflow of 

 blood from the arteries into the capillaries and veins is enormous. 

 For, as the blood passes into innumerable small vessels, it 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 

 opposes less friction to the inflow 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, instead of there being a resistance to outflow from 

 the veins into the heart, this is favoured by the suction action 

 of the heart during diastole, and also by the fact that the great 

 veins, in entering the heart, pass into the thorax, an air-tight 

 box in which during each inspiration a very low pressure is 

 developed. 



What has been said of the veins applies equally to the 

 lymphatics. 



2. Rhythmic Variations in Blood Pressure 



Before considering the exact measurements of pressure . in 

 these different vessels, certain rhythmic variations in pressure 

 may first be considered. 



A. Synchronous with the Heart Beats 

 Arterial Pulse 



With each ventricular systole about 80 grms. of blood are 

 thrown into the already full arteries, and the pressure in these 

 vessels is suddenly raised. 



