310 THE HUMAN BODY 



maintained. During diastole the ventricle would fill from the 

 veins, and during systole empty into the arteries. But in order 

 to accomplish this, during the systole the valves at the point of 

 entry must be closed, or the ventricle would empty itself into the 

 veins as well as into the arteries ; and this closure would necessitate 

 a great loss of time which might be utilized for feeding the pump. 

 This is avoided by the auricles, which are really reservoirs at the 

 end of the venous system, collecting blood when the ventricular 

 pump is at work. When the ventricles relax, the blood entering 

 the auricles flows on into them; but previously, during the part of 

 the cardiac cycle occupied by the ventricular systole, the auricles 

 have accumulated blood, and when they at last contract they send 

 on into the ventricles this accumulation. Even were the flow from 

 the veins stopped during the auricular contraction this would be of 

 comparatively little consequence, since that event occupies so 

 brief a time. But, although no doubt somewhat lessened, the 

 emptying of the veins into the heart does not seem to be, in health, 

 stopped while the auricle is contracting. The heart in fact con- 

 sists of a couple of " feed-pumps " the auricles and a couple of 

 "force-pumps" the ventricles; and so wonderfully perfect is the 

 mechanism that the supply to the feed-pumps is never stopped. 

 The auricles are never empty, being supplied all the time of their 

 contraction, which is never so great as to obliterate their cavities; 

 while the ventricles contain no blood at the end of their systole. 



The Work Done by the Heart. According to the physical 

 definition work is measured by the weight lifted times the height 

 to which it is raised. In estimating the work of the heart we sub- 

 stitute for the height the resistance against which the heart works. 

 This resistance is equivalent in the case of the left ventricle to that 

 of a column of blood about 2 meters high, and for the right ven- 

 tricle about 0.8 meter. The mass of blood ejected from each ven- 

 tricle during systole probably averages about 100 gms. The 

 work done by the left ventricle per beat equals, then, about 

 100x2 = 200 grammeters, and that by the right ventricle equals 

 about 100x0.8=80 grammeters. Since the heart in addition to 

 moving the weight of blood imparts to it a considerable velocity, it 

 is necessary to add to the amounts of work calculated above an 

 additional amount to represent that required to impart to the 

 blood its velocity. This latter amount approximates 3 gram- 



