CHAPTEE XXI 



THE CIRCULATION IN THE BLOOD-VESSELS 



THE movement of the blood from the heart through the arteries, 

 capillaries, and veins back to the heart again, depends on a number 

 of physical factors; and in the consideration of this important subject 

 we shall have to take into account the general laws which regulate 

 the movement of fluids in tubes, as well as their special application 

 to the flow of the blood in the blood-vessels. 



The contraction of the heart is the primary propelling force, and 

 the increase of pressure which is thus communicated to the blood it 

 contains causes that blood to enter the arteries ; the arterial blood 

 pressure is higher than that in the capillaries, and the capillary 

 pressure is higher than that in the veins; the venous pressure 

 gradually falls as we approach the heart ; it is lowest of all in the 

 heart cavities during diastole ; fluid moves in the direction of lower 

 pressure, hence the flow of blood is from the heart through the 

 vessels back to the heart again. 



The vessels are not rigid tubes, but possess marked elasticity ; it 

 is owing to this that the intermittent force of the heart is modified 

 in such a way that the stream of blood in the capillaries is a constant 

 one, and under normal circumstances exhibits no pulsation ; the pulse 

 is one of the main characters of the arterial flow. A further com- 

 plication is due to the fact that the vessels through which the blood 

 flows are of varying calibre, and this is the main factor in determin- 

 ing its velocity. Every time an artery divides, the united sectional 

 area of its branches is greater than that of the parent artery, although, 

 of course, each of the individual branches is of smaller calibre. The 

 total bed of the stream is thus becoming greater, until when we 

 reach the capillaries the bed is increased suddenly and enormously, 

 being several hundred times greater than that of the aorta from 

 which they all ultimately spring. In the case of the veins the same 

 is true in the reverse direction ; the sectional area of a vein is less 

 than that of the total sectional area of its tributaries ; hence as we 



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