30 PHYSIOLOGY FOR NURSES 



blood will immediately flow but a short distance, but 

 the force will be applied to every drop of blood in every 

 artery. Each artery will expand and contract just as 

 the aorta does and the expansion will be synchronous 

 with the heartbeat; but that particular jet of blood will 

 not reach the artery being felt for several seconds. It 

 follows from this that the greater the power of the 

 heartbeat the stronger will be the pulse; the more fre- 

 quent the heartbeat, the more rapid the pulse, the more 

 the capillaries are expanded, the less the resistance to 

 the outflow, the more compressible the pulse and the 

 lower the blood pressure. 



In the lungs the blood circulates under the same gen- 

 eral conditions as in other parts of the body. Pressure 

 is less in the pulmonary vessels because the right ventri- 

 cle is weaker than the left. Inspiration and expiration 

 also affect both the systemic and pulmonary circulation. 

 In inspiration about one-twelfth of all the blood in the 

 body is in the lungs, while expiration reduces this to 

 one-fifteenth or eighteenth. 



Blood Pressure. If liquid be pumped into an inelas- 

 tic tube, the pressure will be exactly proportionate to 

 the quantity of fluid and the force of the pump. As soon 

 as the pump ceases to act, the pressure falls to zero. If 

 the same experiment be conducted with an elastic tube 

 pressure will again be equal to force and quantity, but 

 the elastic recoil of the tube will maintain some pressure 

 after the pump ceases to act. If the tube be converted 

 into two whose combined area is as great as that of the 

 single tube, but with thinner walls, pressure will de- 

 crease because friction will be greater ; and if the proc- 

 ess of division be continued until each tube is of micro- 

 scopic size, but their combined area is much greater than 

 that of the original tube, pressure will be very greally 



