CHAPTER XI 



THE CIRCULATION OF THE BLOOD AND OF THE 

 LYMPH 



Nearly every one has seen firemen handling hose, and has 

 noticed how, as the engine pumped, water spurted out at the 

 couplings or at some leak in the hose, showing that it was 

 under great pressure. This pressure is due both to the steady 



pumping of the en- 

 gine and to the small 

 nozzle at the end of 

 the hose. If the noz- 

 zle were taken off, the 

 water would run in a 

 large stream but 

 would not be thrown 

 any great distance, 

 as there would be 

 nothing to prevent 



the whole pipeful from escaping as fast as the water was 

 pumped into the hose. In such a case the water would be 

 under little or no pressure; Fig. 85. 



BLOOD PRESSURE AND ITS CAUSE 



The blood in our arteries is under pressure for similar rea- 

 sons, and the pressure is produced by two similar factors, (1) 

 the heart beat, and (2) the resistance offered by the capillaries 

 to onward flow. Since the heart contracts about seventy 

 times a minute and pushes fresh blood into the aorta, the 

 influence of its beat is evident. The narrower arteries and 

 capillaries offer great resistance to the blood flow, corre/ 



FIG. 85. SHOWING THE EFFECT UPON PRES- 

 SURE OF DIFFERENT-SIZED NOZZLES 

 There is a leak in each hose and the height of the 

 stream of water shows the pressure. 





