MUSCULAR WORK AND CARDIAC OUTPUT 283 



The main fault to be found with this calculation is that the 

 various quantities required are almost impossible to obtain. For 

 instance, the only methods by which the output of the heart in 

 situ can be determined are indirect. Zuntz, by finding the 

 percentage amount of oxygen which the blood gains per unit of 

 time in passing through the lungs, and the actual amount of 

 oxygen taken from the lungs per unit of time, calculated the 

 amount of blood that had passed through the lungs during that 

 period. For example, if the blood gains 5 per cent, of oxygen 

 and the lungs part with 30 c.c. of oxygen to the blood, then, in 

 order to have a 5 per cent, mixture, 600 c.c. of blood must have 

 passed through the lung in unit time. Now if the heart beats 

 70 times per minute, and the unit of time chosen was 1/5 of a 

 minute, then the volume of the right ventricle would be 



5 X 600/70 =almost 43 c.c. 



Since, of course, the left and right ventricle must discharge equal 

 amounts of blood, the output of the left ventricle is found. 



Muscular work causes an increase in the output per beat. 

 Under resting conditions, it is probable that the amount of blood 

 entering the heart during the diastole is not sufficient to fill the 

 ventricle up to the limits set by the fibrous inextensible bag 

 surrounding the organ (pericardium). The first effect of the call 

 for more oxygen set up by the muscles is to increase the output 

 of the heart per beat. The power of the heart to thus increase its 

 capacity is limited. By a reflex mechanism the heart rate is 

 increased and so the output per minute is augmented. The 

 following table shows approximately the share of the burden 

 of increasing the output borne by increased distension of the 

 ventricular walls and by increased pulse rate. 



TABLE XLIX. 



EFFECT OF WORK ON CARDIAC OUTPUT. 



It will be seen that at first the pulse rate is practically unaltered 

 although the amount of work done has been increased from 270 to 



