THE NUTRITION OF THE HEART 165 



blood, the amount of blood necessary to carry away 1,000 c.c. i 

 ^- - =10,000. If this be divided by the beats per minute (100), 



then the output for each beat would be 100 c.c. In man, the output 

 volume can be determined by breathing in a deep breath of air mixed 

 with nitrous oxide a very soluble gas holding the breath and find- 

 ing how much of this gas is carried away by the blood during a 

 given time, about thirty seconds. A sample of the alveolar air is 

 taken at the beginning and at the end of holding the breath, and the 

 amount of air in the lungs at the beginning and at the end of this 

 period is determined. From the amounts of nitre: s oxide in the 

 samples the amount of nitrous oxide present in the lungs at the 

 beginning and at the end of the experiment is known. This gives 

 the amount absorbed from the lung in the time. Then, knowing the 

 solubility of the gas (1 c.c. of blood absorbs 0-43 c.c. of the gas), the 

 amount of blood necessary to absorb the known amount from the 

 mean percentage of gas present can be calculated. This divided by 

 the length of time of the experiment and the heart-beats per minute 

 gives the output for each beat. The output for man is calculated to 

 be 60 to 100 c.c. It is ten times as great or more during hard exercise. 

 The work of the heart may be calculated from the following 

 formula : 



.W-H + *', 



where M= the weight of the mass of blood moved, and gr=the acceler- 

 ating force due to gravity, this = 9-8. Q x H represents the 

 work of each heart-beat in overcoming the peripheral resistance. 



Mv 2 



=the energy of the velocity of the blood ejected. These two 

 *ff 

 must be added together. 



We may take the output of the left ventricle as 100 grammes ; the 

 mean pressure of the aorta as 110 mm. Hg. Since mercury is about 

 13-5 times heavier than blood, this 



= 110 millimetres x 13-5, or 

 0-110 metre x 13-5. 



We have therefore in gramme-metres of work : 



inn v fi.^i 2 

 W= 100x0-110 x 13-5+ - 



2 x 9-8 



= 148-5 +1-26,5 

 = 149-765, 



or, approximately, 150 gramme-metres of work for each contraction 

 of the left ventricle. It is clear that almost all the work of the heart 

 is spent in overcoming resistance, and it suffices, for roughly calcu- 

 lating the work, to multiply the output by the arterial pressure. 



We can say that the work of the left ventricle is equivalent to- 



