CH. XXI.] THE CIRCULATION IN THE BLOOD-VESSELS. 26 1 



CHAPTER XXL 



THE CIRCULATION IN THE BLOOD-VESSELS. 



THE circulation through the vessels is accomplished by the 

 heart as the primary propelling force ; the pressure in the heart- 

 is greater than that in the arteries ; the arterial pressure (which 

 is kept high not only by the heart's force, but by the existence 

 of resistance at the periphery) is greater than that in the capil- 

 laries, and the pressure is lowest in the veins, especially at their 

 entrance into the heart ; and fluid always flows in the direction 

 of lower pressure. Before, however, passing on to the all-important 

 question of blood-pressure, we must first consider various other 

 phenomena in connection with the flow in the vessels, such as the 

 velocity of the stream, and the character of the flow in different 

 parts of the vascular circuit. 



The Velocity of the Blood-Flow. 



The velocity of the blood-current at any given point in the 

 various divisions of the circulatory system is inversely propor- 

 tional to their sectional area at that point. If the sectional area 

 of all the branches of a vessel united were always the same as 

 that of the vessel from which they arise, and if the aggregate 

 sectional area of the capillary vessels were equal to that of the 

 aorta, the mean rapidity of the blood's motion in the capillaries 

 would be the same as in the aorta ; and if a similar correspondence 

 of capacity existed in the veins and arteries, there would be an 

 equal correspondence in the rapidity of the circulation in them. 

 But when an artery divides, the sectional area of its branches is 

 greater than that of the artery from which they originate. The 

 only exception to this rule is seen in the division of the aorta into 

 the two common iliac arteries. It is the same with the veins ; 

 the sectional area of a vein formed by the union of smaller veins 

 is less than the total sectional area of its tributaries. From the 

 aorta onwards to the capillaries there is a gradual increase of the 

 area of the stream with a corresponding diminution of its velocity ; 

 from the capillaries onwards to the heart there is a gradual decrease 

 of the bed of the stream and a corresponding increase in its velocity. 



In other words the arterial and venous systems may be repre- 

 sented by two truncated cones with their apices directed towards 

 the heart; the area of their united base (the sectional area 



