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Mechanical and Electrical Character of the Heartbeat /9 : 2 



reaches the capillaries, and the pressure fluctuations are smoothed out. 

 As the blood enters the venous system, the pressure is still lower. Just 

 before the blood enters the heart, the gauge pressure is negative; because 



Q = AV 



A = na 



a = cross section of vessel 

 A = total cross section 

 v = linear velocity 

 Q — volume flow rate 



Figure I . Linear velocity of the blood. Since the volume flow 

 rate, Q, remains approximately constant throughout the cir- 

 culatory system, a low linear velocity, v, means a large cross 

 section A. In the capillaries, the vessel cross sections, a, are 

 small, but the number in parallel, n, is so large that A is 

 greater in the capillaries than in the arterioles or veins. After 

 G. H. Best and N. B. Taylor, The Physiological Basis of Medical 

 Practice, 7th ed. (Baltimore, Md. : Williams and Wilkins Com- 

 pany, 1961). 



Figure 2. Variation of blood pressure at fixed time, 

 shown are gauge pressure in a normal, adult human. 



Values 



