



THE CAUSATION OF THE HEART BEAT 1001 



Only when the functional continuity of the heart muscle is impaired by 

 the * block ' effect of vagal stimulation or by the administration of muscarine 

 is it possible to obtain phenomena even superficially analogous to the sum- 

 mation of contractions in skeletal muscle. 1 



FACTORS MODIFYING THE ACTIVITY OF CARDIAC MUSCLE 

 INFLUENCE OF TENSION AND DISTENSION 



When we examine the behaviour of a heart isolated from the central 

 nervous system and from the rest of the body, as for instance in the heart- 

 lung preparation (vide p. 955), we find that it has a marvellous power of 

 adaptation, i. e. of regulating its activity according to the mechanical demands 

 which are made upon it. Thus while we may maintain the venous inflow 

 constant so that the heart is sending out a litre of blood per minute, it makes 

 no difference to the output of the heart whether the average arterial pressure, 

 and therefore the resistance to the outflow of blood, be maintained at 80 or 

 160 mm. Hg., although in the latter case the heart must do. exactly twice as 

 much work in order to maintain the outflow at the same level. Again 

 if we maintain the arterial pressure constant and alter the venous 

 inflow, we find that within very wide limits the heart is able to expel 

 against the. arterial resistance the whole of the blood which flows into it from 

 the veins. In this way we can alter the output of a small heart of 50 gms. 

 from 300 to 3000 c.c. per minute. As we should expect, this variation in 

 the work done by the heart is associated with corresponding variations 

 in the chemical changes which occur at each heart beat. Evans has shown 

 that the respiratory exchanges of the heart increase pari passu with the 

 work it has to do. Thus in an isolated dog's heart, weighing 70 grms., with 

 a constant inflow and output of 35 litres per hour, raising the arterial pressure 

 from 80 mm. Hg. to 140 mm. Hg. increased the oxygen consumption from 

 228 to 404 c.c. per hour. In another experiment with a heart of 59 grms., 

 in which the arterial pressure was maintained constant at 80 mm. Hg., 

 increasing the output from 9-3 to 92 litres per hour raised the oxygen con- 

 sumption from 155 to 649 c.c. per hour. In these experiments the maximum 



/ work done in calories \ 



efficiency of the heart -, - , v ; -, : ) varied between 20 and 



Vtotal metabolism in calories/ 



30 per cent., and was of the same order as that found for voluntary muscle. 

 Careful investigation of the volume and pressure changes of the heart 

 der varying conditions of arterial resistance and venous filling enables 

 s to throw some light on the mechanism of this power of adaptation. 

 Let us take first, the changes in volume as recorded by the cardiometer. 

 A heart is contracting 100 times per minute and forcing out at each beat 

 10 c.c. of blood into the aorta against an average pressure of 80 mm. Hg., 

 with systolic and diastolic pressures respectively of 100 and 60 mm. Hg. In 

 rder that the left ventricle may force 10 c.c. of blood against this resistance, 



1 According to Mines the effect of vagus excitation in enabling the production of 

 summation is due to the shortening of the refractory period which results from vagal 

 stimulation. 



