CIRCULATION 287 



B. FLOW OF BLOOD 



The flow of blood, as already indicated, depends upon the 

 distribution of pressure, a fluid always tending to flow from 

 the point of higher pressure to the point of lower pressure. 

 Since a high pressure is maintained in the aorta and a low 

 pressure in the veins entering the heart and in the cavities of 

 the heart during its diastole, the blood must flow through the 

 circulation from arteries to veins. (Practical Physiology.} 



Velocity 



The velocity of the flow of a fluid depends upon the width 

 of the channel. Since in unit of time unit of volume must 

 pass each point in a stream if the .fluid is not to accumulate 

 at one point, the velocity must vary with the sectional area 

 of the channel. In the case of a river, in each second the 

 same amount of water must pass through the narrowest and 

 through the widest part of its channel. Now for a ton of 

 water to get through any point in a channel one square yard 

 in sectional area in the same time as it takes to pass a point 

 in a channel ten square yards in area, it must obviously 

 flow with greater velocity. This may be stated in the pro- 

 position that the velocity (V) of the stream is equal to the 

 amount of blood passing any point per second (v) divided by 

 the sectional area of the stream (S) 



where S is the radius squared multiplied by the constant 314. 



In. the vascular system the sectional area of the aorta is 

 small when compared with the sectional area of the smaller 

 arteries ; while the sectional area of the capillary system is 

 no less than 700 times greater than that of the aorta. In 

 the venous system the sectional area steadily diminishes, 

 although it never becomes so small as in the corresponding 

 arteries, and where the great veins enter the heart it is about 

 twice the sectional area of the aorta (fig. 132). 



This arrangement of the sectional area of the stream gives 

 rise to a rapid flow in the arteries, a somewhat slower flow in 

 the veins, and to a very slow flow in the capillaries. 



