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. 382, 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 6 than 

 at a. The same dependency 



of velocity on the total bed 



must apply in any closed a 



system of tubes. Thus in a & 



closed circuit (Fig. 381) with 



a steady flow from 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 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 



999 



