212 THE CIRCULATION OF THE BLOOD 



ever, a well-established fact that the bloodflow in the central part of the 

 circulation is more or less reciprocal with that at the periphery, an 

 increase in the one place being accompanied by a corresponding de- 

 crease in the other. 



The Visceral Bloodflow in Man 



The visceral bloodflow in man can be measured indirectly in the case 

 of the lungs, either, (1) by finding the quantity of oxygen absorbed by the 

 blood during an interval of time that is less than that required for the 

 blood to travel once round the circulation (60 seconds) and comparing 

 this with the oxygen content of samples of arterial and venous blood, or (2) 

 by causing a person to breathe a known quantity of nitrous-oxide gas and 

 then finding how. much is taken up by the blood in the lungs. In the for- 

 mer method the difference in oxygen percentage between arterial and 

 venous blood will be less for a given absorption of oxygen from the alveoli 

 the more rapid the circulation of blood through the lungs; in the latter 

 method, the absorption of a given amount of nitrous oxide will be pro- 

 portional to the rapidity of the bloodflow. Obviously these estimations 

 must be made only over periods of time, less thai; that taken for any of 

 the blood to complete one circuit of the circulation. 



Since it is likely that such measurements will find some application in the study 

 of cardiovascular disease, it may be well to indicate briefly how they are carried out. 

 In the oxygen absorption method, the amount of this gas that is absorbed from the 

 lungs in one minute is determined by analysis of inspired and expired air. The ar- 

 terial blood is considered as saturated with oxygen, and the percentage of this gas in 

 the venous blood (entering the lungs) is computed by using the dissociation curve. 

 Suppose 1,000 c.c. of t> 2 was absorbed in one minute* and that the arterial blood con- 

 tained 10 per cent more O 2 than the venous, then, since each 100 c.c. of blood carried 

 away 10 c.c. of gas it would take 10,000 c.c. to carry away the 1,000 c.c. of O actually 

 absorbed; and suppose the pulse to be 80 per minute then, with each heart beat 



10,000 



= 125 c.c. of blood must have flowed through the lungs. In the nitrous oxide 



80 



method 1 ** the' person inspires a deep breath of a mixture of this gas and oxygen from a 

 meter, and after holding the breath for a few seconds (to allow the gas to mix uni- 

 formly with the alveolar air) he expires sufficiently to bring the lungs to their mid 

 position and again holds the breath for about 30 seconds, after which he finally expires 

 forcibly. Samples of the expired air are taken from the last portions of the two expira- 

 tions, and the percentage of nitrous oxide in them determined. From the percentages 

 of nitrous oxide found in the two samples, the amount of the gas in the air of the lungs 

 at the beginning and at the end of the period can be estimated. Suppose that 55 c.c. 

 N O was found to be absorbed in 0.327 minutes, that the percentage of N O is 11.08 

 then, since the absorption coefficient of N^O in blood at 37 C is 0.405 (i. e., 1 c.c. of 

 blood dissolves 0.405 c.c. N o O at 37C., Krogh and Lindhurd the amount of blood re- 



*The average consumption of an adult at rest is only about 230-250 c.c. of Oo, the above value 

 of 1,000 c.c. being however observed during muscular exercise. 



