346 THE HUMAN BODY 



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 little or 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- 

 meters. The total work output of the heart per beat is, therefore, 

 roughly 283 grammeters, equivalent in the English scale to about 

 2 foot-pounds. When the heart is beating at the rate of 70 per 

 minute it does 140 foot-pounds per minute, making it a 240th 

 horse-power engine. If it maintained this rate throughout the 

 entire twenty-four hours of the day it would do in that time 

 200,000 foot-pounds of work, an amount equivalent to that done 

 by the leg muscles of a man weighing 150 pounds in climbing a 

 mountain 1,300 feet high. 



That the heart is able to do this amount of work daily without 

 fatigue, and keep it up day in and day out for seventy or more 

 years, is due to its ability to recover quickly from the effects of its 

 activity, coupled with the fact that in a whole day its resting time 

 considerably outweighs the time during which it is active. The 

 heart-beat is ordinarily much slower during sleep than during 

 bodily activity; as the result the heart enjoys an "eight hour day" 

 if only its actual contraction time be counted. 



