PRESSURE DEVELOPED IN VENTRICLES 373 



during the isometric phase. The heart responds to increased work 

 by such a lengthening of its fibres. If the lengthening process is 

 carried too far, the muscle fibres per unit of area will become fewer, 

 so that the larger the ventricular volume, the more strongly will 

 each fibre have to contract in order to produce a given tension. At 

 this greater length they also use up more potential energy just as 

 skeletal muscle does. 



We have seen (Chap. XIV.) that when skeletal muscle contracts 

 about two-fifths of the energy used is actually converted into 

 tension. If all the tension energy were then converted into 

 external work, the mechanical efficiency of this type of muscle 

 would be about 40 per cent. The realisable efficiency differs from 

 this theoretical value, because, even provided the load and rate 

 are optimal (q.v.), a considerable amount of energy is rendered 

 unavailable for work because it is dissipated in overcoming the 

 resistance of the viscous muscle to shortening. The more rapid 

 and the more complete the shortening, the greater will be the 

 amount of energy lost. The optimum efficiency is obtained when 

 the muscle pulls on a load that is always optimal, i.e. varies so as 

 to be always as great as the muscle can move. At the beginning 

 of contraction the load should be great, and it should gradually be 

 decreased as the shortening process proceeds. 



This desideratum is found in the heart. As soon as the ventricles 

 start to empty, the shortening cardiac muscles have a steadily 

 decreasing mass of blood to act against. 



It is of further interest to note that during severe muscular 

 exercise optimal conditions are found for cardiac efficiency, i.e. a 

 high output at moderate arterial pressure. Under these circum- 

 stances the efficiency of the heart is about 26 per cent. (cf. Muscle). 



Form and Function 



Pressure Developed in Ventricles. The diagrammatic section of 

 the heart (Fig. 85) demonstrates that the walls of the left ventricle 

 are much thicker than those of the right. The mean of a large 

 number of determinations furnishes the ratio of 6-8 : 1. This may 

 be interpreted as indicating that the left ventricle develops six to 

 seven times as much pressure as the right ventricle. Proof 

 confirmatory of this deduction is obtained by determining the 

 hydrostatic pressures necessary completely and symmetrically to 

 fill these two chambers. The right ventricle is dilated by a seventh 

 of the pressure employed in equally dilating the other ventricle. 



A dog weighing 10 kilos with an average aortic pressure of 100 mm. Hg, 

 and an output of 2,000 c.c. of blood per minute, develops pressure in right and 

 left ventricles of 25 and 150 mm. Hg respectively — a ratio of 25 : 150 = 1:6. 



