164 



Mechanical and Electrical Character of the Heartbeat /9 : 4 



person hears through a stethoscope. The two can be made quite similar 

 by differentiating the sonograph output twice. 



Aortic 

 Pressure 



Ventricular 

 Pressure 



^i- Auricular 

 Pressure 



Ventricular 

 Volume 



Electro- 

 cardiogram 



Heart 

 Sounds 



Systole 



Diastole 



Figure 5. Pressure sequences in the left side of the heart. The 

 significance of the vertical lines is as follows : 1 . the mitral valve 

 closes; 2. the semilunar valve opens; 3. the systolic pressure 

 reaches a maximum; 4. the semilunar valve closes; 5. the mitral 

 valve opens; 6. end of heart sound; and 7. the auricle starts to 

 contract. After C. H. Best and N. B. Taylor, The Physiological 

 Basis of Medical Practice, 7th ed. (Baltimore, Md. : Williams 

 and Wilkins Company, 1961). 



B. Electrical Events 



The heart pulses rhythmically and with a definite sequence. The beat 

 is initiated at the sino-auricular (s-a) node, shown in Figure 4. The 

 node acts in a fashion similar to a relaxation oscillator putting out an 

 electrical pulse (about every -^ of a minute in man). This pulse spreads 



