28 PHYSIOLOGY FOR NURSES 



pact and the elastic recoil both lose power until, in mi- 

 nute arteries the blood flow approaches the character of 

 a steady stream instead of the jet-like type of the larger 

 arteries. In the arteries the blood leaps in jets ; in the 

 capillaries it oozes; in the veins it flows. It is like a 

 swift torrent which spreads into a marsh where move- 

 ment is barely perceptible until its outlet is found in a 

 deep, dirty, sluggish stream. 



The venous current is collected by the veins from the 

 capillaries. The blood, through capillaries and veins, 

 still receives an impulse from the heart, but not in suf- 

 ficient strength to complete the return circulation. Cer- 

 tain accessory forces are, therefore, called into play. 

 The first of these is the suction of the thorax caused by 

 inspiration, and described under that head. A more 

 widespread factor is that of the valves in the veins, par- 

 ticularly those so located that gravity habitually retards 

 the flow. These valves are half cup shaped, concavity 

 upward, very much like the semilunar valves, so ar- 

 ranged that the blood flows easily over their free sur- 

 face, but is caught in the hollow cup when it attempts 

 to fall back. From this position it is forced on to the 

 next valve partly by the push of the heart from behind, 

 partly by suction of the thorax in front and largely by 

 the massage-like action of the muscles which squeeze 

 and compress the veins and drive the blood to the next 

 valve, where the same process is repeated. 



Circulation Time. The circulation time can not be 

 definitely stated. The usual estimate is that from twen- 

 ty-five to thirty beats of the heart are required to com- 

 plete the circulation ; i.e., to drive the blood throughout 

 the body. About a fourth of this number completes the 

 pulmonary circulation. The velocity through different 

 vessels also varies as does the quantity supplied to each 



