214 THE CIRCULATION OF THE BLOOD 



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 isotohic 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 a'dmit 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, 

 and more susceptible than that of the lungs to different conditions of 

 temperature. In a dog in which the pulmonary circulation time was 

 about 8.5 seconds, that of the spleen was about 11 seconds, and of the 

 kidney about 17.5 seconds. The shortest circulation time of all is of 

 course that in the coronary artery, but that through the retina can not fall 

 far behind it. 



To determine the total circulation time, we must know: (1) the average 

 amount of blood passing by each part in -a given time, and (2) the average 

 circulation time of each part. From such computations, which however 

 are obviously subject to considerable error, it has been reckoned that the 

 total circulation time in man must lie somewhere between 1 and 1.25 

 minutes. 



MOVEMENT OF BLOOD IN VEINS 



Before leaving this part of our subject, a few words may be said con- 

 cerning the forces concerned in the movement of blood in the veins from 



