9 : 4/ Mechanical and Electrical Character of the Heartbeat 163 



lungs. Thus, the blood in a complete circuit goes through the heart 

 twice, once through the left side and once through the right side. This 

 system is highly efficient in supplying oxygen and removing carbon 

 dioxide, for all the blood passes through the lungs on each trip around the 

 circulatory system. 



The walls of the heart consist primarily of muscle tissue. As in all 

 other striated muscles, the fiber membranes are normally polarized, the 

 inside being 90 mv negative relative to the outside. Just before con- 

 traction occurs, an action current passes over the membrane, reversing 

 its polarity for a short period of time. The form and nature of these 

 action currents are similar to those of nerve fibers discussed in Chapter 4. 

 The large mass of fibers contracting simultaneously in the heart effect- 

 ively acts as a large number of electric cells, all in parallel, and each with 

 a high internal impedance. Although the net current from each fiber 

 is small, the current from the entire muscle is appreciable, giving rise to 

 measurable potential changes on the body surface. 



4. The Heart Sequence 



A. Over-all Sequence 



The mammalian heart pumps blood with uniform sequence which 

 repeats each beat. First the auricles contract, forcing blood through the 

 auriculoventricular (a-v) valves into the ventricles. Then the ventricles 

 contract. This shuts the a-v valves and opens the semilunar and pul- 

 monary valves. As the ventricles continue to contract, blood is forced 

 into the aorta and pulmonary arteries. Finally, as the ventricles relax, 

 the semilunar and pulmonary valves close. The entire sequence is 

 presented in more detail in Figure 5, which shows, with a common time 

 base, the auricular pressure, the ventricular pressure, the aortic pressure, 

 and the ventricular volume for a human heart. Also, on the same base 

 are shown the electrocardiograph (ekg) record and the sonograph 

 record of a microphone placed against the chest. Heart pressures have 

 been measured directly in both man and animals. The ventricular 

 volumes have been found by X-ray techniques. 



From the diagram, it is clear that the blood flows from the ventricle 

 into the aorta only during a small part of the cycle. While this is 

 happening, the ventricular volume falls to a minimum value, but the 

 pressure remains close to its maximum. Likewise, an examination of the 

 figure shows that the valves open and shut as the direction of the pressure 

 difference across them changes. The sonograph obtained by putting a 

 broad-band microphone on the chest is strikingly different from what a 



