1 14 CIRCULATION 



the pulse is spoken of its a die rot ic />///.sr. This is apt to ucconi- 

 pany a low blood /;/r,v,v///r. The dicmfic innr y.v canned In/ a 

 sudden tension of the semilunar /v///v.v. 



The blood moves through the arteries in a series of pulses 

 which grow less and less pronounced, until they are extinguished 

 in the capillary district. Here the Mood (lows toward the 

 veins with much friction, slowly and under comparatively 

 low pressure. Instruments have been devised to measure 

 rapid fluctuations of speed. They consist essentially of a needle, 

 which is thrust through the wall of the vessel. The amount 

 that ir> deflected from the perpendicular by the movement 

 of the blood is read on a graduated semicircle which is placed 

 under the free end of the needle. It has been ascertained that 

 the blood in the large arteries flows at a rate of from 2">() to 

 over 500 mm. a second. The speed in the veins is somewhat 

 slower. In the capillaries it has been measured directly under 

 the microscope, and some physiologists have observed it in 

 the retinal capillaries of their own eyes. It flows from (U> 

 to 0.9 mm. a second. The speed and pressure of the blood rise 

 and fall together in the arteries, but whereas the pressure 

 falls continuously from arteries to veins, the speed falls from 

 arteries to capillaries, but is increased again in the veins as 

 it approaches the heart. The speed does not depend upon the 

 pressure alone, but also upon the width of the blood path. 

 Whenever a vessel divides, the cross-sectional area of its branches 

 is greater than that of the vessel itself. The collccfin' sec- 

 tional area of the capillaries is several hundred times that of 

 the aorta, while the latter is one-half that of the vena* ca\a\ 

 The blood then flows swiftly through the arteries to the capil- 

 laries, where it performs its functions and is returned almost 

 as quickly to the heart by the veins. It has been estimated 

 that the blood remains about ().(> of a second in a capillary 

 .1 mm. long. 



The pulmonary circulation differs in minor respects from 

 the systemic. The total friction is less, in correspondence 

 with which the right ventricular walls are far thinner than those 

 of the left ventricle. Owing to the fact that the pulmonary 

 system lies entirely within the thorax, it is subjected to the 

 negative pressure which exists there, and the veins and arteries 

 are opened by the clastic pull of the lungs. This tends to favor 



