THE MECHANISM OF THE HEART PUMP 915 



the other hand, the output is at once altered by alterations in the venous 

 inflow and, as already stated, can be altered in a heart weighing 50 gms. 

 from a few c.c. up to 3000 c.c. per minute. The only essential in this 

 preparation is that the output from the left ventricle shall be sufficient 

 to maintain a circulation through the coronary vessels and so keep the active 

 muscle properly supplied with blood. 



With increasing inflow of blood into the heart the large veins, auricles, 

 and ventricles naturally become more filled during diastole, and during 

 systole of the ventricles, when the auriculo-ventricular valves are closed, 

 the blood rushing in from the venous system must accumulate in the big 

 veins and auricles to a still greater extent. The venous pressure therefore 

 rises with increased venous inflow. In so far then as venous pressure is 

 an index of venous inflow, we may say that the output of the heart increases 

 with the venous pressure so long as the heart is functionally capable of 

 dealing with the blood it receives during diastole. But although the 

 ventricular output is practically independent of the frequency of the heart- 

 beat and a constant venous inflow, the venous pressure tends to fall as 

 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 the 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 : 



