372 CIRCULATION 



TABLE LIX 



Efficiency of the Heart under various Conditions 



so used multiplied by 2-07 gives, in kilogram-metres, the energy 

 developed. It is clear that, with a moderate increase in arterial 

 resistance, the mechanical efficiency of the heart improves, but 

 tends to decrease when the resistance is doubled. In other words, 

 when the arterial pressure is raised, the oxygen intake is increased, 

 and more tension developed in the cardiac muscle. The mechanical 

 efficiency is raised to a certain limit, beyond which it again 

 diminishes. The venous pressure in the experiment quoted, and in 

 most others, runs parallel with the oxygen usage. In the series of 

 observations tabulated as B, the arterial pressure was kept constant 

 at about 80 mm. Hg, while the output per hour was increased 

 roughly as 1 : 2 : 3. This was done by varying the inflow of blood 

 to the heart. The increase in oxygen usage is not quite propor- 

 tional to the increase in work done, but is, if anything, less. The 

 efficiency values, therefore, tend to increase with increasing outputs 

 up to a certain limit. Beyond this point, the amount of oxygen 

 used increases very suddenly. In the example given, for a little 

 less than double the output, almost two and a half times as much 

 oxygen is required. As this involves the liberation of enough 

 energy to lift 1,343 kg. to the height of a metre, and as only 

 126-3 kilogram-metres of work are done, the increased work is not 

 done so economically and therefore the efficiency value falls. 



Maximal Efficiency. How can this primary increase in efficiency 

 and subsequent decrease be explained, and what factors are brought 

 into play to settle the critical point at which maximal efficiency will 

 be found ? If output is to be increased, intake must first be 

 increased and the ventricle must be distended to hold the extra 

 amount of blood. That is, the muscle fibres of the ventricular wall 

 will be stretched. We have already mentioned, in connection with 

 skeletal muscle, that a stretched muscle develops more tension 



