THE MECHANISM OF THE HEART PUMP 1031 



In a similar experiment on a dog the output per second of the right ventricle 

 was found to be '00157 of the body-weight. In order to get the output at each 

 beat it will be necessary to divide the output per minute by the number of 

 heart-beats in the same time. 



From the results of these various determinations on animals it 

 has been calculated that the output of the right ventricle in man at 

 each beat is equivalent to between 50 and 100 c.c. and may be taken 

 on an average at 60 c.c. The output of the left ventricle must be 

 exactly equal to that of the right ventricle, otherwise there would 

 be a damming up of the blood at some part or other of the circulation. 

 The output depends chiefly on the diastolic filling of the heart. It is 

 therefore increased by any factor which increases the latter, such as 

 rise of venous pressure or lengthened diastole, such as occurs during 

 enforced slowing of the heart-beat. 



THE WORK OF THE HEART 



The energy of the ventricular contraction is expended in two ways : 

 firstly, in forcing a certain amount of blood into the already dis- 

 tended aorta against the resistance presented by the arterial blood- 

 pressure, which itself is directly conditioned by the resistance in 

 arterioles and capillaries ; and secondly, in imparting a certain velocity 

 to the mass of blood so thrown out. Thus the energy of the muscular 

 contraction is converted partly into potential energy in the form of 

 increased distension of the arterial wall, and partly into the kinetic 

 energy represented by the momentum of the moving column of blood. 

 The work done at each beat may be calculated from the formula : 



where W stands for work, w for the weight, and Q for the quantity 

 (volume in c.c.) of blood expelled at eacji contraction : R is the 

 average arterial resistance or pressure durk '/'the outflow of blood from 

 the heart, and V is the velocity of the blood at the root of the aorta. 

 In this equation QR is the work done in overcoming the resistance,* 



~\72 



and - is the energy expended in imparting a certain velocity to the 

 2g 



blood. 



If we take 60 c.c. as the average output of each ventricle, 100 mm. 



* This expression, QR, is only approximately correct. Supposing the pressure 

 in the aorta at the beginning of systole is 50 mm. Hg. and at the end of systole 

 150 mm., the work could not be deduced accurately from the average pressure, 

 but would need a simple application of the integral calculus for its determina- 

 tion. The expression employed above deviates from the real value only by 

 about 10 per cent., and is therefore sufficiently accurate for our purpose. 



