THE MECHANISM OF THE HEART PUMP 957 



recorded on a travelling surface, so that the duration of the experiment is represented 

 by the horizontal distance between the lines showing the moments of sampling (Fig. 

 416). 



By comparison of the composition of ordinary alveolar air with the alveolar air ob- 

 tained after the first sharp expiration, the amount of residual alveolar air is determined, 

 so that the total volume of gas contained in the lungs at each part of the experiment 

 is also known. During the time when the breath is being held, nitrous oxide is being 

 taken up in solution by the blood as it passes through the lungs, its solubility being such 

 that 1 c.c. of blood, if exposed to an atmosphere of pure nitrous oxide, will. take up 

 0'43 c.c. of this gas. From the data obtained in this way, the amount of blood passing 

 through the lungs during the period between the two expirations can be calculated. The 

 following record of one experiment may serve as an example. The volume of air in 

 the lungs at the beginning of the experiment was 3-25 litres and contained 12 per cent, 

 nitrous oxide, so that the total quantity of nitrous oxide in the air of the lungs was 

 3250 c.c. X Y^% = 390 c.c. At the end of the period the total volume of air in the 

 lungs was three litres, containing only 10 per cent, nitrous oxide, so that the lungs 



\ 

 \ 



267560 



FIG. 416. (KROGH.) 



now contained only 300 c.c. nitrous oxide, 90 c.c. nitrous oxide having been taken up 

 by the blood. This 90 c.c. was taken up from an air in which the mean pressure of 



this gas was " =11 per cent. During the period of observation, from a gas 



2 



containing at atmospheric pressure 11 per cent, of nitrous oxide, each c.c. of blood 

 will take up - = 0-047 c.c. In order to take up 90 c.c. therefore, 1-9 litres of 



-Lv/v/ 



blood must have passed through the lungs during the time of the observation. The 

 experiment lasted twenty-eight seconds. The amount of blood passing through the 

 lungs per minute was therefore 4-2 litres. This figure represents the output from the 

 right ventricle during one minute, and if the pulse rate is 70 per minute, the output 



4200 c c 

 per beat will be = 60 c.c. per beat. The figure, arrived at in this way for the 



average output of each ventricle in man during rest, thus agrees with the figure obtained 

 by Zuntz. The output of both ventricles is of course the same. 



According to Krogh, the ventricular output per minute in man may vary from 2-8 

 litres to 21 litres of blood per minute. The latter is an extreme figure and was obtained 

 in a powerful athlete doing hard work. In the case of Krogh himself, the maximum 

 output was about 12 litres per minute. It is interesting to note that the same perform- 

 ance may be obtained from a dog's heart in the heart-lung preparation, allowing for 

 the difference in size between the hearts of the dog and man respectively. 



CARDIOMETRIC METHOD. Of the various methods which have been devised for 

 recording plethysmographically the changes in the volume of the heart at each beat (as 

 first carried out by Roy), the simplest is that devised by Henderson. The chest and 

 pericardium being opened, a glass cardiometer, of the shape shown in Fig. 417, is slipped 

 over the heart. This cardiometer consists of a glass sphere with a wide opening. To 

 the margin of the opening is tied a rubber diaphragm with a hole in it, which accurately 

 fits the heart as it lies in the auriculo-ventricular groove. The tube of the cardiometer is 



