554 CIRCULATION OF BLOOD AND LYMPH. 



augment the flow of venous blood into the heart. On the basis of 

 100 c.c. one may make an approximate estimate of the work done 

 at each beat. Using Tigerstedt's figures, such results as the follow- 

 ing are obtained: On the left side the heart empties its 100 c.c. 

 against a pressure of 15 cm. Hg. and on the right side against a 

 pressure of, say, 6 cm. Hg. The work done may be calculated from 

 the general formula W^FS— that is to say, the work W is equal to 

 the product of the force F into S, the space or distance through 

 which the force acts. If the left ventricle empties 100 c.c. of blood 

 at each systole into the aorta, then, since the diameter of the aorta 

 is, say, 3 cm., there is forced out of the ventricle a column of blood 

 about 14 cm. long with a cross area of 7 sq. cm. This column is 

 driven out of the ventricle against a pressure or resistance equiva- 

 lent to the weight of a column of mercury 15 cm. high, or, counting 

 the specific gravity of blood as 1.06, a column of blood 192 cm. high 

 (15 X 7%?). This is equivalent to saying that at each systole the 

 left ventricle lifts a column of blood 192 cm. long and with a cross 

 area of 7 sq. cm. to a height of 14 em. A column of blood 192 cm. 

 high with a cross area of 7 sq. cm. weighs 1425 gm. (192X7X 1.06). 

 To move this weight through a distance of 14 cm. means the per- 

 formance of 19,950 gm. cm. of work (1425X14). In addition to 

 this work which represents the overcoming of a resistance there is 

 the further work represented by the velocity communicated to the 

 blood discharged from the ventricle. The kinetic energy of this 

 moving mass may be calculated from the formula: 



^ = T- 

 Substituting for m its value of -, in which p represents the weight 



and g the acceleration of gra\dty (p = mg, hence m = ?j,then 



E = 2^- in which p is equal to 106 gm. and v to 50 cm., the average. 



velocity of the blood in the aorta. ^^^^ ^ = 135 gm. cm. of 



work. The total work done by the left ventricle, therefore, at each 

 systole is equal to 20,085 gm. cm. It will be noted that according 

 to this calculation the velocity factor constitutes less than 1 per 

 cent, of the total energy of the ventricular contraction (135 : 

 20,085). Most of the energy is used up in overcoming the re- 

 sistance. But when the output from the heart is increased, as in 

 muscular exercise, the importance of the velocity factor is mag- 

 nified, a greater part of the energy of the contraction is used in 

 giving velocity to the discharged blood, and the circulation is 

 correspondingly hastened.* The calculations made by different 

 * Evans, "Journal of Physiology," 52, 6, 1918. 



