HEART 413 



unit of time, 30 c.c. of oxygen are taken up by the lungs, 

 then this 30 c.c. must be distributed in the blood to the 

 extent of 5 c.c. for each 100 c.c. of blood (fig. 177), and 

 hence 600 c.c. of blood must have passed through the lung 

 in unit of time. 



2. The Nitrous Oxide Method, — Instead of estimating the 

 increase in the oxygen of the blood, a measured quantity 

 of nitrous oxide, NgO, the solubility of which in the blood 

 at the pressure and temperature at which it is present in the 

 lungs is known, may be inhaled into the lungs and estimate 

 may be made of — 



(1) The amount of NgO in the arterial blood. (2) The 

 amount of NgO which has been taken from the lungs by the 

 blood per unit of time. 



From this the amount of blood passing through the lungs, 

 i.e. from the risjht ventricle, mav be calculated as it is bv the 

 oxygen method. 



Krogh has shown that the left ventricle in man pumps 

 blood into the arteries at the following rates according to 

 the condition of muscular activity : — 



1. At rest about 3 litres per minute. 



2. With moderate exercise ,, 12 ,, ,, 



3. With hard „ „ 21 



In the horse the quantities are probably greater. 



The Average Work. — It is thus impossible to attempt to 

 form an estimate of the average work of the heart 

 since these variations are so great. 



The Adaptation of the Work. — A more interesting question 

 is — How is the heart able to adapt itself to perform the 

 very different amounts of work required ? This has been 

 elucidated by Starling by the use of the isolated heart-lung 

 preparation (fig. 178). 



(i.) The heart and lungs are carefully removed from a 

 dog, all the vessels being clamped or ligatured, (ii.) The 



