THE CIRCULATION 



235 



to the blood leaving it may be directly recorded by means 

 of a lever attached to the piston. 



The average resistance in the aorta may be taken at about 

 1-5 metres of blood. Hence, with each beat, the left ventricle 

 may perform 0-08 x 1'5 = 0-12 Kgms. of work. The right 

 ventricle is only one-third as 

 strong as the left, and hence 

 the work done by each beat 

 is only 0*04 Kgms. 



If the heart is beating 72 

 times per minute, the amount 

 of work per minute will be 

 something under 11 '5 Kgms., 

 or 16,560 Kgms. in 24 hours. 

 Some investigators estimate 

 it as low as 10,000 Kgms. 



In cardiac muscle the 

 greater the resistance to con- 

 traction the stronger the force 

 of contraction. Hence, when 

 extra blood is poured into the 

 heart from the veins, or when 

 the outflow from the ventricles 

 into the arteries is impeded, 

 the increased strain put upon 

 the heart muscle is met by 

 increased contraction, and the 

 additional work thrown upon 



the organ is effectually performed. Not only is this the 

 case when temporary disturbances of the circulation occur, 

 but when these disturbances are permanent, the heart adapts 

 itself to them, and, if it has continuously to perform extra 

 work, its muscular wall hypertrophies, just as the skeletal 

 muscles grow by continual use. Of course, to allow such 

 compensation to be established, the blood supply to the 

 heart muscle must be sufficient, and hence, when the 

 coronary arteries are diseased, heart failure rapidly ensues. 

 If the coronary arteries are clamped and then relaxed, 

 a peculiar fibrillar contraction of the heart muscle 

 occurs. 



FIG. 116. Eoy's Cardiometer to 

 measure the output of blood from 

 the heart. b, heart in cardio- 

 meter chamber ; c, piston re 

 corder working on lever against 

 rubber band, d. 



