SECTION III 



THE VELOCITY OF THE BLOOD AT DIFFERENT 

 PARTS OF THE VASCULAR SYSTEM 



WHEN fluid is flowing through, a tube of uniform diameter, the amount 

 passing between any two points is practically in proportion to the difference 

 of pressure between these two points, and varies inversely as the resistance 

 to be overcome. If the tube is of unequal bore, as represented in Fig. 397, 

 since the amount of fluid passing a during a given interval of time must be 

 equal to the amount passing 6 where the bed of the stream is wider the 

 velocity of the flow must be smaller at b than at a. The same dependency 



of velocity on the total bed must . . 



apply in any closed system of | | 



tubes. Thus in a closed circuit 



(Fig. 396) with a steady flow from a L_ I 



the arterial to the venous side, * 



the amount of fluid leaving the 



heart and passing A during a minute must be exactly equal to the total 



amount of fluid passing from arteries to veins through the peripheral 



resistance B. 



The total area at c is probably one thousand 'times that of the aorta at A, 

 and we should expect therefore a proportionate slowing of the blood stream. 

 As a matter of fact, while the velocity of the blood in the aorta of a large 

 animal may be taken as about half a metre per second, the velocity of the 

 blood in the capillaries is about half a millimetre per second. Moreover, 

 since the total cross-section of the big veins near the heart under a normal 



I distending pressure is about twice that of the first part of the aorta, the 

 velocity of the blood in the great veins is only about half of that found in 

 the aorta. In such a closed circuit increased output of the heart will increase 

 the average velocity round the system, and the same effect may be produced 

 by diminution of the peripheral resistance. 



In the living body a great dilatation of the arterioles, causing a fall of 

 the peripheral resistance, generally increases the total capacity of the system. 

 The arterial relaxation therefore not only gives rise to an easier outflow 

 from arteries to veins but also causes a diminished dilatation of the veins, 

 and therefore decreased filling of the heart during diastole. The heart 

 output is therefore also lessened, so that a final result of a dilatation of the 

 arterioles may be a diminished instead of an increased velocity throughout 



the system. 



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