CIRCULATION OF THE BLOOD. 49 



Age. Before birth, the number of 'pulsations per minute averages 140 



During the first year it diminishes to 128 



During the third year diminishes to 95 



From the eighth to the fourteenth years averages 84 



In adult life the average is 72 



Exercise and digestion increase the frequency of the heart's action. 



Posture influences the number of pulsations per minute ; in the male, 

 standing, the average is 81 ; sitting, 71; lying, 66; independent, for the 

 most part, of muscular effort. 



The Rhythmical movements of the heart are dependent upon I. An 

 inherent irritability of the muscular fibre, which manifests itself as long as 

 the nutrition is maintained. 2. The continuous flow of blood through its 

 cavities, distending them and stimulating the endocardium. 



The force exerted by the left ventricle at each contraction has been 

 ^estimated at 52 pounds. If a tube be inserted into the aorta, the pressure 

 there will be sufficient to support a column of blood nine feet or a column 

 of mercury six inches in height, the weight in either case being about four 

 pounds. The estimation of the force which the heart is required to exert 

 to support this column of blood, is arrived at by multiplying the pressure 

 in the aorta (4 pounds) by the area of the internal surface of the left 

 ventricle (about 13 inches). Each inch of the ventricle being capable of 

 supporting a downward pressure of 4 pounds. 



Work done by the Heart. The work done by the heart is estimated by 

 multiplying the amount of blood sent out from the right and left ventricles 

 at each contraction, by the pressure in the pulmonary artery and aorta 

 respectively, e. g. t when the right ventricle contracts, it forces out one- 

 quarter pound of blood, and in so doing must overcome a pressure in the 

 pulmonary artery sufficient to support a column of blood three feet in 

 height; that is, must exert energy sufficient to raise ^ Ib. 3 feet, or % X 3 

 or % Ib. one foot. When the left ventricle contracts, it sends out % Ib. ol 

 blood, and in so doing, the left ventricle must overcome a pressure in the 

 aorta sufficient to support a column of blood nine feet in height; that is 

 must exert energy sufficient to raise J^ Ib. 9 feet, or ^ X 9 or 2 X ^ s - one 

 foot. Work done is estimated by the amount of energy required to raise 

 a definite weight a definite height, the unit, the foot pound, being that 

 required to raise one pound one foot. 



The heart, therefore, at each systole exerts energy sufficient to raise 3 foot 

 pounds, and as it contracts 72 times per minute, it would raise in that 

 time 3 X 7 2 or 216 foot pounds ; and in one hour 216 X or 12,960 foot 



