244 



PHYSIOLOGY OF THE HEAKT 



[CH. XX. 



Frequency of the Heart's Action. 



The heart of a healthy adult man contracts about 72 times in a 

 minute ; but many circumstances cause this rate, which of course 

 corresponds with that of the arterial pulse, to vary even in health. 

 The chief are age, temperament, sex, food and drink, exercise, time 

 of day, posture, atmospheric pressure, temperature. Some figures 

 in reference to the influence of age are appended. 



The frequency of the heart's action gradually diminishes from the 

 commencement to near the end of life, but is said to rise again some- 

 what in extreme old age, thus : 



Before birth the average number 



of pulsations per minute is . 150 

 Just after birth . . from 140 to 130 

 During the first year . ,, 130 to 115 

 During the second year ,, 115 to 100 

 During the third year . 100 to 90 



About the seventh year . from 90 to 85 

 About the fourteenth 



year , 85 to 80 



In adult age . . . 80 to 70 

 In old age . . 70 to 60 



In decrepitude . . ,, 75 to 65 



In health there is observed a nearly uniform relation between 

 the frequency of the beats of the heart and of the respirations ; the 

 proportion being, on an average, 1 respiration to 3 or 4 beats. The 

 same relation is generally maintained in the cases in which the action 

 of the heart is naturally accelerated, as after food or exercise ; but 

 in disease this relation may cease. 



Work of the Heart. 



Waller compares the work performed by the heart in the day to that done by an 

 able-bodied labourer working hard for two hours. The heart's work consists in dis- 

 charging blood against pressure, and in imparting velocity to it. Thus, if V repre- 

 sents the output of the heart per beat measured in cubic centimetres, and P the 

 mean pressure in the aorta, m the mass of the blood, and v the velocity imparted to 

 it ; the work W is given by the equation : 



W= VP + \ mm 2 - 

 = Vgdh + * mv> 



where h is the mean pressure in the aorta expressed in centimetres of blood, 

 d the density of the blood, and g the acceleration of gravity (981). 



If now a is the transverse section of the aortic orifice, b that of the aorta, t the 

 duration of the ventricular systole, and ^ the duration of the cardiac cycle, then, if 

 x is the mean velocity of the blood in the aorta, 



V = avt = bv^. 



Let us assume that the output of the heart is 110 c.c. per beat. The duration 

 of the cardiac cycle is 0-8 sec., and that of the ventricular systole is 0-3 sec. The 

 diameter of the aorta is about 3 cms. and that of the aortic orifice 2 '6 cms. Remem- 

 bering that the radius in each case is half the diameter, we have : 



Therefore 



110 = 7r(l'3) 2 x 0-3 x v = 7r(l'5) 2 x 0'8 x v l 

 v = 86-03, and ^ = 19 '45 cms. per second. 



That is, the velocity of the blood as it is discharged from the heart is about 4-5 times 

 greater than the mean velocity of the blood in the aorta. 



If H represents the mean intra ventricular pressure during the time blood is 



