212 TIIK CIRCULATION OF THE BLOOD 



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 oilier. 



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 nitrons-oxide gas and 

 then finding the concentration of this gas in the blood after leaving the 

 lungs. In the former method the difference in oxygen percentage be- 

 tween 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, and in the latter method for the absorption of a given amount 

 of nitrous oxide, the less will be the concentration of this gas in the 

 blood the more rapid the circulation. Obviously these estimations must 

 be made only over periods of time, less than that taken for any of 

 the blood to complete one circuit of the circulation. 



The methods are admittedly only approximate, but the results are of 

 much interest, mainly because of the indication they give us as to the 

 amount of blood pumped out by the ventricle with each heartbeat, or 

 during a given period of time. The results have been found to vary 

 considerably; thus, one author (Krogh) places the output of blood per 

 minute as between 2.3 and 8.7 liters, which would correspond, at a 

 pulse rate of 70, to an output per heartbreat of from 40 to 120 c.c. An 

 immediate and very marked increase has been found to occur during 

 muscular work. By comparing the bloodflow through the hand with 

 that through the lungs, an estimate can be formed in a given individual 

 as to the relative magnitude of the peripheral and visceral moieties of 

 blood. Interesting results, which will be referred to later, have been 

 obtained from such measurements. 



The Work of the Heart 



Meanwhile it is of interest to note that we may calculate from the 

 ventricular output of the blood the amount of work that the heart is doing 

 in maintaining the circulation. Of course the calculation is again only 

 approximate, since we have to assume certain figures. If we assume that 

 in a 70-kilogram man the quantity of blood is 4,200 c.c. (see page 85), 

 and that it takes about one minute for all the blood to complete a cir- 

 culation, then the work performed by the left ventricle in one minute 



