CHAPTER VI 

 THE CIRCULATION OF THE BLOOD 



THE SYSTEMIC CIRCULATION 



The Velocity of the Blood 



Whenever an artery divides, the branches, though 

 individually smaller than the parent-trunk, have collectively 

 a larger area of cross-section. The combined area of cross- 

 section of the capillaries is many hundred times greater 

 than that of the aorta. Similarly, as the veins converge, 

 the total area of the tributaries becomes smaller. Blood 

 is therefore flowing away from the heart in a stream which 

 is ever widening, and back to the heart in a stream which is 

 ever narrowing. On this account the velocity of the blood 

 diminishes as it travels along the arteries, reaches its 

 minimum in the capillaries and quickens again in the 

 veins. 



We have no means of measuring the velocity of the blood 

 directly in the human subject. An indirect calculation can, 

 however, be made of the rate at which it travels through 

 the aortic orifice. The output of the left ventricle per beat 

 while the body is at rest we have seen to be on an average 

 60 c.c. At a pulse rate of 72, this gives 4320 c.c. per 

 minute. The area of cross-section of the aorta is 4 sq. 

 cm. In one minute, therefore, a column of 1080 cm. 

 passes along the aorta. Were the flow continuous, this 

 would give a velocity of 18 cm. per sec. 



Many instruments have been invented for measuring 

 the velocity of the blood in animals. For use in arteries 



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