

THE MECHANISM OF THE HEART PUMP 959 



the heart beat increases in rate. The optimum venous pressure is that 

 which fills the ventricle during its diastole to the maximum extent to 

 which it is able to respond. As the rate of the heart increases, the inflow 

 of blood can also be increased without causing over-distension of the 

 ventricles. The increase of heart rate therefore is an important factor 

 in enabling this organ to deal with the maximum amount of blood. Although 

 increase of rate does not alter the output with constant venous inflow, it 

 does increase the maximum amount of inflowing blood which the heart is 

 able to expel. 



We thus see that alterations in the vigour of the circulation depend in 

 the first instance on the venous circulation. The greater volume of the 

 blood that is brought up to the. heart by the accessory factors of the cir- 

 culation, the greater will be the output of this organ. The changes in rate 

 and force of the heart which accompany its increased activity and increased 

 output, e. g. during exercise, represent merely the means by which this 

 organ is able to deal in the most advantageous manner with the increased 

 inflow. 



THE WORK OF THE HEART 



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

 first, in forcing a certain amount of blood into the already distended 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 each contraction; R is the average arterial 

 resistance or pressure during 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 



"\72 



the work done in overcoming the resistance, 1 and - is the energy expended 



2 # 

 in imparting a certain velocity to the blood. 



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

 as the average pressure at the beginning of the aorta, and 500 mm. per 



1 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 determination. The expression 

 employed above deviates from the real value by at most 10 per cent., and is therefore 

 sufficiently accurate for our purpose. 



