BATE OF MOVEMENT OF THE BLOOD 213 



will be equal to that done in raising the above quantity of blood to a 

 height corresponding to the mean pressure in the aorta. If we take this 

 pressure as 130 millimeters of mercury, which would correspond to -a 

 column of blood 1,755 meters high (13.5 x 130=1755 mm. blood, or 1.755 

 meter), the work done by the left ventricle would be 1.755x4.2=7.37 

 kilogram-meters in one minute, or in twenty-four hours roughly about 

 10600 kilogram-meters. The work done by the right ventricle is probably 

 about one-third that of the. left, this being about the ratio of the pres- 

 sures in the two chambers. The total work of the two ventricles is there- 

 fore about 14000 kilogram-meters. This represents an enormous amount 

 of work; indeed it has been computed that it is sufficient to raise a man 

 of 70 kilograms to about twice the height of the highest skyscraper in 

 New York. The work thus expended in forcing the blood through the 

 capillaries becomes converted by friction in the small blood vessels into 

 heat, the heat equivalent of the above amount of work being roughly 

 about 350 calories (see page 537). 



THE CIRCULATION TIME 



The circulation time, or the time taken by a drop of blood to travel 

 between two points in the circulation, can be determined in laboratory 

 animals by a variety of methods, all depending on the principle of seeing 

 how long it takes for a drop of some substance injected into an artery to 

 appear in the corresponding vein. For example, to determine the time 

 taken for a drop of blood to pass from the jugular vein into the carotid 

 artery in a rabbit, a solution of methylene blue in isotonic saline is in- 

 jected into the former vessel and the moment of its appearance through 

 the walls of the artery determined by a stop-watch. If the walls are too 

 thick to admit of the employment of this method, a strong solution of 

 sodium chloride may be substituted for the methylene blue, and the mo- 

 ment of its appearance at another point of the circulation determined by 

 observing the electrical conductivity of the vessel. Since the con- 

 ductivity of a blood vessel depends partly on the concentration of elec- 

 trolytes in the blood flowing through it, the moment at which the salt 

 solution appears will be indicated by a change in electrical resistance 

 (G. N.Stewart). 



By such methods, it has been found that the time for the pulmonary 

 circulation is very short compared with that of the systemic circulation. 

 In a rabbit it is usually a little less than four seconds; in an average- 

 sized dog of about 12 kilograms, it is about eight seconds; and in man 

 it is computed to be about fifteen seconds. On the other hand, the cir- 

 culation time in such viscera as the spleen and kidney is relatively long, 



