306 



THE MECHANICS OF THE HEART 



period of contraction of the heart lasts 0.4 sec, and that of relaxation 

 and rest 0.4 sec.^ Under ordinary conditions, therefore, this organ 

 rests as much as it works. Greater frequencies are attained at the 

 expense of the pause until eventually even the amplitude of the con- 

 traction is lessened. The preceding discussion, however, will show 

 that a frequency of 120-140 per minute may be obtained by simply 

 dropping the pause. 



E. THE PLAY OF THE VALVES 



It must be clearly understood at this time that the movement of 

 the valve flaps depends upon the differences in pressure to which their 



Fig. 157. — The Intraventricular (VP) Pressure in Relation With the Intra- 

 AURicuLAR Pressure (AP) and the Heart Sounds (S). 

 AB, auricular systole; BD, auricular diastole; DA, auricular pause; BC, ventricular 

 systole; CE, ventricular diastole; EB, ventricular pause; 1, closure of auriculo ventricu- 

 lar valve; 2, opening of semilunar valve; 3, closure of semilunar valve; 4, opening of 

 auriculo ventricular valve; I, II and ///, first, second and third heart sounds. 



upper and lower surfaces are exposed. To begin with, the blood flows 

 into the diastolic auricles until it finally sets up a pressure which is 

 just sufficient to push the auriculoventricular valve flaps downward. 

 A part of the auricular contents are now enabled to escape into the 

 relaxed ventricles. The mitral and tricuspid valves, therefore, open 

 sometime before the onset of auricular systole, and permit the ven- 

 tricular cavities to become partially filled even before the auricles 

 actually begin their contraction (Fig. 157). Thus, the succeeding 

 auricular systole {AB) merely serves the purpose of forcibly filling the 

 ventricles until they are fully distended. Immediately following this 

 phase, the auricles relax (BD), while the ventricles contract {BC) and 

 develop a pressure far in excess of that prevailing in the ante-chambers. 

 The auriculoventricular valves now close (1), and naturally, their closure 



1 More specific values are given in Tigerstedt's Physiologic d. Kreislaufes, Leipzig, 

 1893. 



