262 RESPIRATION 



indirectly calculate how far the gain and loss vary under different 

 conditions. A rough method devised by Yandell Henderson for 

 measuring the relative rates of the blood flow was used in the 

 Pike's Peak expedition, and served to indicate that the rate of 

 blood flow remained practically normal in spite of the great alti- 

 tude. Another method, the principle of which was tried, though 

 without success, by Henderson on Pike's Peak, was about the same 

 time independently worked out and extensively used by Krogh 

 and Lindhard at Copenhagen. 3 This method gives absolute and not 

 merely relative results. The principle of the method is that the 

 lungs are filled by a very deep breath with a mixture containing a 

 considerable percentage of nitrous oxide, a gas which is very solu- 

 ble in blood. A sample of alveolar air is taken after an interval of 

 five seconds to allow the lung tissue to become saturated with the 

 nitrous oxide, and after a further interval during which the breath 

 is held, another alveolar sample. By determining the nitrous 

 oxide in the two samples, and also the total volume of gas in the 

 lungs, we find out how much nitrous oxide has been absorbed. 

 Knowing the solubility of nitrous oxide in blood, and assuming 

 also that the blood leaving the lungs is fully saturated with nitrous 

 oxide to the existing partial pressure of the gas, we can calculate 

 from the loss of nitrous oxide how much blood has passed through 

 the lungs in the given time interval. The experiment must be 

 carried out so rapidly that the venous blood continues to be free 

 of nitrous oxide. 



There are various sources of probable error in this method, but 

 in the hands of Krogh and Lindhard it gave fairly consistent 

 results. They found that during rest the amount of blood circula- 

 ting through the lungs of an adult man varies from about 2.8 

 to 5 liters per minute, and that the arterial blood loses about 30 

 to 60 per cent of its available oxygen on an average, and during 

 considerable work about 50 to 70 per cent. The following table 

 gives calculated volumes of blood passing through the lungs, and 

 calculated percentage losses in the available oxygen of the blood 

 as it passes round the tissues. 



It will be seen that, allowing for the fact that the haemoglobin 

 of arterial blood is only 95 per cent saturated with oxygen, the 

 haemoglobin of the venous blood was apparently only 38 per cent 

 and 53 per cent saturated in the two resting experiments. The 

 flow of blood through the lungs during work appeared to be as 



1 Krogh and Lindhard, Skand. Arch. /. Physiol., XXVII, p. 100, 1912. 



