THE MECHANISM OF THE HEART PUMP 913 



be necessary to divide the output per minute by the number of heart-beats in the same 

 time. From the results of determinations made in this way Zuntz concluded that the 

 output of the right ventricle in man at each beat varies between 50 and 100 c.c. and 

 may be taken on an average at 60 c.c. 



KROGH'S METHOD. In Krogh's method an endeavour is made to determine 

 the volume of blood flowing through the lungs in a given time by determining how 

 much nitrous oxide is taken up from a mixture of nitrous oxide and air, with which 

 the lungs are filled. Nitrous oxide is chosen because it can be breathed in considerable 

 proportions without injury, and is itself very soluble in water or in the blood. The 

 estimation is carried out in the following way. A small recording spirometer is filled 

 with about 4 litres of a gas mixture containing 10 to 25 per cent. N 2 and 20 ,to 25 

 per cent, oxygen. The subject, seated in a chair or on a bicycle ergometer, empires to 

 the greatest possible extent, and then takes a deep inspiration from the spirometer. 

 He holds his breath for five to fifteen seconds, breathes out sharply into the spirometer, 



2-24 



28 '/sec 



FIG. 406. (KROGH.) 



expiring at least one litre. At the end of this sharp expiration, a sample of his alveolar 

 air is taken by connecting the tube from his face-piece with an evacuated glass bulb, 

 as in Haldane's method of determining alveolar air. The breath is now held for a 

 period, time varying between six and twenty-five seconds. He then makes a final sharp 

 ample expiration into the spirometer, a sample of his alveolar air being taken at the 

 end of this expiration. The excursions of the spirometer indicate exactly what volume 

 of air he has breathed in and breathed out at each part of the experiment. These are 

 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. 496). 



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 1 ^ = 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 

 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 



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



