THE ENERG Y OF THE CIRCULATION. 43 



the result arrived at being that the velocity of output is ten times the 

 mean velocity in the aorta. To obtain the output of the left ventricle — 



Q=Avt 



= 7rr 2 x 320x0-1 

 = 5x320x0-1 

 = 160 0.0.! 



The diameter of the pulmonary orifice being reckoned as 3 cms., and 

 the output of the right ventricle being the same as the left, the velocity 

 of output at the pulmonary orifice will be in inverse proportion to its 

 sectional area as compared with that of the aortic orifice. 



(3) 2 : (2 - 5) 2 : : 320 : 222 cms. per sec. (velocity of output at pulmonary orifice). 



The kinetic energy of the velocity of output is largely changed into 

 potential energy in the aorta and pulmonary artery. During diastole 

 the potential energy stored up by the distension of the elastic arterial 

 walls becomes again kinetic. It is spent in maintaining the velocity of 

 How, and in overcoming the viscosity of the blood in the arterioles and 

 capillaries. 



To estimate the work of the heart, the mean pressure and velocity 

 in the aorta, and the volume of the systolic output must be obtained. 



If W be the work done during systole of the left ventricle in 

 gramme centimetres ; Q, the volume of the output in cubic centimetres ; 

 M, the mass of the output in grammes ; P, the specific gravity of blood 

 (. • . M = PQ) ; V, the mean velocity in the aorta ; H, the mean aortic 

 pressure in grammes per centimetre ; g, the acceleration due to gravity 

 = 981 cms. per second; 



Then, W=QH + ^!- 



Now, the mean aortic pressure may be taken to be 12 cms. of 

 mercury (sp. gr. of mercury = 13 '5). The volume of the systolic output 

 has been reckoned above as 160 c.c. ; but experimental determinations 

 of the output in animals lead to the conclusion that the output in man 

 does not much exceed 100 c.c. (see p. 49). For the purposes of this 

 calculation a mean value of 110 c.c, may be taken as the output. 

 Substituting these data in the above equation, one obtains — 



W= 110 x 12 x 13-5 + 1 ' 05 x 110x322 = 17,880 gramme-centimetres. 



2 x 981 



It is obvious that the work spent in maintaining the aortic velocity 

 is almost negligeable (1 : 300 nearly) compared with that spent in over- 

 coming resistance. 



If in the case of the right ventricle the mean pressure in the pul- 

 monary artery be taken to be 4 cms. of mercury, the work of that 

 ventricle will be one-third of that of the left ventricle. 



Thus the total work of each systole of the heart will 1 »e — 



4 

 17,880 x - = 23,640 gramme-centimetres, 

 o 



and the total work of the heart per diem about 24,000 kilogramme- 



1 This is, however, undoubtedly too high an estimate (see p. 49). 



