THE VELOCITY OF BLOOD-FLOW 



3. If both the energy of the heart and the peripheral resistance 

 increase proportionately, the pressure will rise, but the velocity remain 

 constant. 



4. With an increase in the energy of the heart-beat, and a decrease 

 in resistance proportional to the increase of cardiac energy, the pressure 

 will remain constant and the Telocity become greater. 



5. If the energy of the heart decreases, but the peripheral resistance 

 increases in proportion, again the pressure remains constant, but the 

 velocity becomes less. 



6. With a decrease in heart-beat, and a proportional decrease in 

 resistance, the pressure falls, but the velocity remains constant. 



7. By virtue of the vaso-motor mechanism (see p. 229), the periph- 

 eral resistance may at the same time be increased in one section 



FIG. 112, CAROTID ARTERY: VELOCITY OF PRESSURE CURVES. 



and Lortet.) 



(Chauveau 



of the arterial system and decreased in another, so that it is possible 

 for the pressure and velocity in the aorta to remain constant, while 

 the velocity is varying in opposite directions in different parts of the 

 vascular system. The velocity in one particular artery, therefore, is 

 no guide to the general condition pertaining in the system, and such 

 velocity bears no absolute relation to the rate of heart-beat or general 

 arterial pressure. 



In the tracing (Fig. 112) are shown the synchronous records of 

 velocity ( V) and of pressure (P) obtained in the carotid of the horse. It 

 will be seen that the curve of velocity reaches its maximum before the 

 curve of pressure. This is so because, as the arteries become overfilled, 

 the heart cannot maintain the initial velocity of output. By experiment 

 it was found that the velocity in the carotid artery of the horse reached 

 520 millimetres per second during systole, while at the time of the 

 dicrotic wave the velocity sank to 220 millimetres per second, and in 

 diastole to 150 millimetres per second. Continuous records of the 

 velocity curve afford a valuable means of arriving at the volume of 

 blood flowing through the vascular area supplied by the artery in 



