9 : 2/ Mechanical and Electrical Character of the Heartbeat 159 



A fluid like the blood may possess both kinetic and potential energy. 

 The kinetic energy per unit volume T is 



T = l P v* 



The potential energy per unit volume V results from both the pressure 

 on the fluid, 1 and its height h above the earth. In physics texts, it is 

 shown that, for an incompressible fluid 



V= pgh+p 



The total energy per unit volume H then is 



H = p + P gh+ \ P v* (1) 



Bernoulli's equation states that H is a constant. It is true only for 

 nonviscous liquids. In general, the variation of H gives the change in 

 energy per unit volume. The blood loses energy for each cycle in the 

 capillaries. The heart, in pumping, increases the energy per unit 

 volume of blood as the latter passes through the heart. Thus, the heart 

 might be called a chemicomechanical transducer. 



When an incompressible fluid flows through a closed system, either 

 the volume flow rate Q (volume per unit time) must be constant at all 

 points or the volume of the system must change. To a first approxima- 

 tion, the average volume of the circulatory system remains constant. 

 Accordingly, the average volume flow rate will usually be the same at all 

 points in the circulatory system. (There are a number of conditions 

 under which more, or fewer, blood vessels are open. For instance, 

 during activity, the blood flow to the muscles increases as more capillaries 

 are open. Similarly, the swelling of erectile tissue is due to expansion 

 of blood sinuses resulting from decreased arteriolar resistance.) 



The variation of blood velocity » in a mammal is diagrammed in 

 Figure 1. Although the arteries and veins are much larger than the 

 capillaries, there are so many capillaries that the total cross-sectional 

 area of the tubes open to the blood is much greater than in the larger 

 vessels. Accordingly, the linear velocity of the blood in the capillaries 

 is smaller than in the arteries and veins. The pulsations in the arteries 

 are possible because the walls are elastic and stretch from the force of 

 each heartbeat. 



In a similar manner, one may diagrammatically represent the pressure 

 variations. These are shown in Figure 2. The maximum arterial 

 pressure is called the systolic pressure, and the minimum arterial pressure is 

 called the diastolic pressure. The pressure falls by the time the blood 



1 Purists will no doubt object to calling p a form of potential energy per unit 

 volume, but this is satisfactory for discussions of the circulatory system. 



