410 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



The circulation time for this particular circuit is: 6.6 seconds in 

 the cat, 7.4 seconds in the rabbit, 16.3 seconds in the dog, and 28.8 

 seconds in the horse. For man the time for the completion of a cir- 

 cuit of medium length has been calculated at 23 seconds so that from 

 26 to 28 beats of the heart are required to effect this journey. In 

 other words, a droplet of blood traverses the circulatory system about 

 three times in every minute. 



More recently, Stewart^ has devised a method which is based upon 

 changes in the electrical conductivity of the blood. The carotid 

 artery is connected with non-polarizable electrodes, the segment be- 

 tween them being inserted as a resistance in one arm of a Wheatstone's 

 bridge. As soon as a balance has been established so that the galvano- 

 meter remains at rest, a solution of sodium chlorid is injected into the 

 external jugular vein of the opposite side. This salt serves the purpose 

 of lessening the resistance of the blood to the electrical current. As 

 soon as this quality of blood arrives at the point designated, the balance 

 in the Wheatstone's bridge is lost and the galvanometric needle is 

 deflected. The time elapsing between the injection and the moment 

 of the deflection is determined by means of a stop-watch or an ordi- 

 nary chronographic appliance. Stewart^ has also employed solutions of 

 methylene-blue which were injected into the external jugular vein and 

 were rendered visible in the opposite carotid artery by means of 

 transillumination upon a white sheet of paper. With the help of the 

 first method, Stewart has also determined the time consumed by the 

 blood in its passage through various organs. In the case of the spleen 

 the average time is given as 10.95 seconds, and in the cases of the kidneys 

 and lungs as 13.3 and 8.4 seconds respectively. These figures show 

 first of all that a considerable part of the total circulation time of the 

 blood must be apportioned to the capillary networks of these organs 

 and secondly, that the time for the pulmonary circuit is relatively 

 short. In man it has been estimated at 12-15 seconds. A still 

 shorter time is required for the completion of the coronary circuit. 

 In this connection, brief mention might also be made of the fact that 

 the circulation time between the portal vein and the arteries amounts 

 to about 12 seconds, and the time between the femoral or renal veins 

 and the arteries to 16 and 13 seconds respectively.* These figures 

 have been obtained by measuring the interval between the injection 

 of adrenalin and the resultant rise in arterial blood pressure. 



1 Jour, of Physiol., xv, 1894. 



2 Manual of Physiol., London, 1896. 



' Burton-Opitz, Am. Jour, of Physiol., xli, 1916, 91. 



