THE VASCULAR MECHANISM. 2OI 



He then holds his breath for twenty to thirty seconds and again 

 expires deeply, a second sample of alveolar air being collected. The 

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

 (experimental) period between the two expirations is determined by 

 indirect means. During this period nitrous oxide is 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 the gas. 



From these data the amount of blood passing through the lungs in 

 a minute can be calculated. To take an example, the volume of air 

 in the lungs at the beginning of the experimental period is 3 '25 litres, 

 and contains, as shown by the analysis of the first sample, 12 per cent, 

 nitrous oxide ; the total quantity of nitrous oxide in the air of the 



lungs is 325Q C - C - X J L2 = 390 c.c. At the end of the period the total 

 100 



volume of air in the lungs is 3-0 litres ; the percentage of nitrous oxide 

 in the second sample of expired air is 10 per cent. ; and the lungs contain 

 300 c.c. nitrous oxide. Thus 90 c.c. nitrous oxide have been taken up 

 by the blood ; and the mean percentage of nitrous oxide in the air in 



' . 12 per cent. + 10 per cent. -. n 

 the lungs is - -=11 per cent. 



With this percentage of nitrous oxide in the air of the lungs, each 

 1 c.c. of blood passing through them will take up ^- = 0*047 c.c. 



nitrous oxide ; and in order to take up 90 c.c., 1'9 litres of blood must 

 have passed through the lungs during the experimental period. If the 

 experimental period lasted twenty-seven seconds, the flow of blood 

 through the lungs per minute is 4'2 litres. This figure represents 

 the output from the right ventricle during that period, and if the 



pulse rate is 70 per minute, the output per beat will be - = 



60 c.c. per beat. This figure may be taken as representing the average 

 output of each ventricle in man, since in health the output of the two 

 ventricles is the same. 



The mean arterial pressure in man is about 100 to 110 mm. Hg. 

 From these data the work done by the heart at each beat can be 

 calculated. Thus Q x R = 60 grm. x 0100 metre x 13*6 (specific gravity 

 of mercury being 13*6 times that of blood) = 81 '6 grm. metres. This 

 figure represents the work done by the left ventricle. If the pressure 

 in the pulmonary artery be taken as 20 mm. Hg, the work done by 

 the right ventricle will be 60 grm. x OO2 metre x 1 3 '6 = approximately 

 16 '4 grm. metres. v 



