136 



PRACTICAL PHYSIOLOGY 



very frequent heat of the pump is required to make the flow con 

 tinuous, and scarcely any fluid passes through the capillary tube. 

 By means of the vaso-motor nerves the arterioles are similarly 

 dilated or constricted, and the current switched on to or off an 

 organ, according to its functional activity. 



Velocity of Flow. (1) Insert the Ludwig stromiihr (Fig. 135) 

 into the artery. It is convenient to fill one side with water, and 

 leave the other full of air. In actual practice one tube is filled 



with defibrinated blood and the 

 other with oil. Set the pump 

 going, and find the number of 

 times the stromiihr must be turned 

 per minute. Turn rapidly the 

 moment the water reaches the 

 mark A^. Each turn means the flow 

 of the quantity of water con- 

 tained in one half of the stromiihr. 

 Measure the capacity of the 

 stromiihr and the diameter of 

 the artery. The capacity of half 

 the stromiihr multiplied by the 

 number of revolutions gives the 

 volume, and this divided by 

 the time and the sectional area 

 of the artery gives the mean velo- 

 city per second. The sectional 

 area of the artery equals the 

 radius x 3*1 4. 



Note the effect on the velocity 

 of (1) opening the clip on the 

 rubber tube, (2) of increasing the frequency of the pump. 



If the energy of the heart is constant, then in proportion as the 

 peripheral resistance increases so the lateral pressure rises and the 

 velocity in the aorta lessens. On the other hand, as the peripheral 

 resistance decreases the pressure falls and the velocity increases. If 

 the peripheral resistance be constant, then as the energy of the 

 heart increases or decreases both the pressure and the velocity in 

 the aorta together become greater or less. By compensatory changes 

 taking place in the heart and the resistance, the velocity may 

 remain constant while the pressure varies, or the pressure may 

 remain constant while the velocity varies. 



The average velocity at any part of the vascular system is inversely 



FIG. 135. The stromUhr. 



