THE VELOCITY OF BLOOD-FLOW 



209 



The Time Necessary for a Complete Circulation. The blood, in 

 leaving the heart, may take a short circuit through the coronary 

 system of the heart, and so back to the right heart, or it may take a 

 long and devious course to the toes and back, or through the intestinal 

 capillaries, portal system, and hepatic capillaries. It is obvious, then, 

 that the time any two particles of blood take to complete the 

 circuit may be different. Experiments have been made to determine 

 how rapidly any substance like a poison, which enters the blood, may 

 be distributed over the body. A salt, such as potassium ferrocyanide, 

 is injected into the jugular vein, and the blood collected in successive 

 samples at seconds of time from the opposite jugular vein. These 

 samples are tested for the presence of the salt by the addition of ferric 

 chloride. A strong solution of methylene blue may be injected into 

 the jugular vein of a rabbit, and the moment determined by a stop- 

 watch when the blue colour appears in the carotid artery. 



By this means the following times were obtained in different 

 animals : 



Squirrel 

 Cat 



Hedgehog 

 Rabbit 

 Dog .. 



Seconds. 

 4-39 

 6-69 

 7-61 

 7-79 

 16-70 



Horse 

 Cock 

 Duck 

 Goose 



Seconds. 



31-50 



5-17 



10-86 



The length of the circuit is found to make little difference in the 

 animal lying horizontal. This is so because the time is chiefly spent 

 in the passage of the blood, not through the arteries and veins, but 

 through the capillaries. Thus, the following are mean results in four 

 double determinations : 



lugular to jugular (dog) 

 Jugular to crural ( dog) 



16-32 seconds. 

 18-08 



If the respiratory movement of a man be recorded, and he take a 

 breath from a bag containing, say, 5-0 per cent, of C0 2 , his breathing 

 will be augmented when the blood charged with excess of C0 2 in the 

 lungs reaches the respiratory centre (half the circuit). This time can 

 be measured, and it approximately indicates the time spent in the 

 blood travelling from the lungs to the centre. 



The circulation time of various organs may be determined by 

 ijecting salt solution into a vein, and observing with the aid of 

 a Wheatstone's bridge arrangement and galvanometer the change in 

 electrical resistance which occurs in the corresponding artery when the 

 salt solution reaches it. The moment of injection and that of the 

 ilteration in resistance are observed with a stop-watch. 



It has been determined that the fastest travelling blood can 

 complete the circuit in about the time occupied by twenty-five to 

 thirty heart-beats say in twenty to thirty seconds. This result 

 shows how rapidly methods must be taken to prevent the absorption 

 )f poisons for example, snake-poison. The fastest travelling blood 

 the pulmonary circuit occupies only about one-fifth of the time spent 

 T that in the systemic circuit. In animals of the same species 



14 



