246 PHYSIOLOGY OF THE HEART. [CH. xx. 



in raising a unit of weight (i Ib.) through a unit of height (i ft.) : 

 in France, the gram-metre. The work done by the heart at 

 each contraction can be readily found by multiplying the weight 

 of blood expelled by the ventricles by the height to which the 

 blood rises in a tube tied into an artery. This height is about 

 i - 5 metres (5 ft.) in man. Taking the weight of blood expelled 

 from the left ventricle at each systole as 87 grammes (3 oz.) 

 and the average pressure in the aorta as no mm. mercury 

 (i'5 metres blood), the work done at each contraction will be 

 130 gram-metres. To this must be added 45 gram-metres for 

 the work done by the right ventricle. If the heart beats 7 2 times 

 a minute, it will do 18,000 kilogramme-metres of work in the 

 24 hours. 



The Output of the Heart. The first estimations of the work of the heart, 

 made by Volkmann and Yierordt. gave numbers nearly double those stated 

 in the preceding paragraph. Recent research has shown that their estimate 

 of the output of the heart was excessive. Direct measurements of the heart's 

 output have been made by Stolnikow and Tigerstedt. The former cut 

 off by ligature the whole of the systemic circulation in the dog, and then 

 measured the amount of blood passing through the simplified circulation 

 which consisted only of the pulmonary and coronary vessels by means of 

 a graduated cylinder interposed on the course of the vessels. Tigerstedt 

 made his observations by means of a Stromuhr (see next chapter) inserted 

 into the aorta. Severe operative measures of this kind, however, interfere 

 with the circulation a good deal. 



Grehant and Quinquand. and Zuntz adopted an indirect method based on 

 the comparison of the amount of oxygen absorbed in the lungs with 

 the amount added to the blood in its passage through the pulmonary 

 circulation. 



G. N, Stewart has introduced an ingenious method, the principle of which 

 is the following. A solution of an innocuous substance, which can be easily 

 recognised and estimated, is allowed to flow for a definite time and at a 

 uniform rate into the heart ; the substance selected was sodium chloride. 

 This mingles with the blood and passes into the circulation. At a conve- 

 nient point of the vascular system, a sample of blood is drawn off just 

 before the injection, and an equal amount during the passage of the salt ; 

 the quantity of the sodium chloride solution which must be added to the 

 first sample in order that it may contain as much as the second sample is 

 determined. This determination gives the extent to which the salt solution 

 has been mixed with the blood in the heart, and knowing the quantity of 

 the solution which has run into the heart, the output in a given time can 

 be calculated. 



All these experiments have been on animals. The results obtained neces- 

 sarily vary with the size of the animal used, and with the rate at which 

 the heart is beating. If the same relationship holds for man as for animals, 

 Stewart calculates that in a man weighing 70 kilo, the output of each 

 ventricle per second is less than 0-002 of the body weight, i.e., about 105 

 grammes of blood per second, or 87 grammes (about 80 c.c.) per heart beat 

 with a pulse rate of 72. Zuntz obtained rather smaller numbers by his 

 method. 



Various methods have been adopted for registering the output of the 

 heart under varying circumstances. A simple cardiometer applicable to 

 the heart of a small mammal like a cat has been devised by Barnard. It 

 consists of an india-rubber tennis ball with a circular orifice cut in one side 



