178 CIRCULATION OF THE BLOOD 



inner surface is 200 mm. X 100 sq. mm. X 13.6 * = 272 g. It is scarcely 

 worth while to give a value for the total power of the left ventricle, since it is 

 not possible to determine with any accuracy the area of its internal surface 

 during systole. In progressive contraction the residual power of a muscle 

 becomes smaller and smaller. But in the heart this is compensated by the 

 fact that at the same time the internal surface of the ventricle is constantly 

 becoming smaller. 



Since the maximal pressure of the right ventricle amounts to about 30 

 mm. Hg., its power per square centimeter of internal surface would be suffi- 

 cient to balance 40.8 g. 



B. WORK 



The work of any chamber of the heart at each systole is expressed by the 

 formula W = pE + ^-, where p is the weight of the output, R is the resist- 

 ance or mean arterial blood pressure maintained by it, v the velocity per 

 second imparted to the blood, and g the acceleration of gravity. 



In order to estimate the work done by the left ventricle, for example, we 

 must determine the pressure and the velocity of the blood in the aorta, as well 

 as the mass of blood driven out at each systole. In the following section w T e 

 shall go into the subject of blood pressure and velocity in the aorta more 

 fully ; here in order to carry out the calculation, we shall say in advance only 

 that the mean pressure may be estimated at about 150 mm. Hg. and the 

 velocity at about 0.5 m. per second. 



We cannot say as yet with any definiteness how great is the quantity of 

 blood expelled from the human heart at each systole. It is very probable that 

 the pulse volume is somewhere between 50 and 100 g. per beat. If we adopt 

 these values and substitute them in the above formula, we obtain as the limits 

 of the work necessary to force the blood against the aortic pressure : 50 X 

 0.150 X 13.6 1 = 102 gram-meters; 100 X 0.150 X 13.6 = 204 g-m. 



The work which it requires to impart a velocity of 0.5 meters to the pulse 



,. , 50x05 a 100x0.5 3 m , 



volume is accordingly -- - = 0.64, or - 7^-5- = 1.28 g-m. The total 

 /c x 9.8 ii> x y.o 



work of the left ventricle in its systole would be therefore 102.64 to 205.28 

 g-m. We see that by far the greater part of the work of the ventricle is used 

 in overcoming the resistance in the vascular system and that only a very 

 small part is necessary to give the blood its mean velocity. This result is 

 perfectly positive in spite of the very arbitrary values used in our calculation, 

 for the pulse volume exercises no influence on the reciprocal relation of the 

 two factors, and even if we estimate the speed in the aorta much higher, and 

 the blood pressure there much lower, the factor pR would still be many times 



greater than the factor ^-. 



We have no direct information as to the quantity of blood expelled from 

 the right ventricle in its systole. But we may assume that its pulse volume 

 is the same as that of the left ventricle; for the left ventricle drives the 

 blood through the greater circulation to the right auricle, and the right drives 



1 The specific gravity of mercury. 



