370 



CIRCULATION 



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 one-fifth 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 ventricles must each discharge 

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



Total Work of the Heart 



Lovatt Evans has shown that if the pressure in the right 

 ventricle be assumed to be one-sixth that of the left, a close 

 approximation to the total work of the heart can be obtained 

 from the expression : 



^y_nJl ^ mjVCf 



6 



gE' 



where 



E = duration of period of expulsion, 

 C — duration of cardiac cycle, and 

 V = mean aortic velocity. 



Stroke Volume. 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 thus to increase its 



TABLE LVIII 



Effect of Work on Cardiac Output 



Muscular work per min. in Kgins. 



Pulse rate per 

 min. 



Output per beat 

 in c.c. 



Output per min. 

 in c.c. 



At rest . . 



Moderate exercise . 270 



Light labour . 735 



Very hard work . 1000 



70 

 100 

 110 

 130 

 180 



45 



75 



120 



115 



117 



3,150 



7,500 



13,200 



14,950 



21,060 



