CIRCULATION OF BLOOD AND OF LYMPH 



Artery 



spending roughly to the nozzle of the fireman's hose. When 

 a large artery is wounded, the force with which the blood 

 comes from the cut strikingly shows this pressure. 



Naturally, the blood in all persons is under some pressure no 

 matter what their age. Age makes much difference, however, 

 and it has been found that in boys and girls of ten years of age 

 the pressure is only about one-half what it is at twenty-five 

 years and again that, in persons of fifty, it is about eighteen per 

 cent more than at twenty-five. Much of this is explained 

 by the decreasing elasticity of the arteries as one grows older. 



In the aorta near the heart the pressure is very consider- 

 able, but it is slight where the blood flows into the capillaries. 

 It will be remembered that the cap- 

 illary walls are very thin, only the 

 thickness of one cell for the most 

 part; and this condition, which is 

 necessary for the ready exchange of 

 materials through their walls, would 

 make it impossible for them to 

 resist high pressure. 



An idea of the amount of pressure 

 in the arteries has sometimes been 

 gained in the following way. If a 

 glass U-tube be inserted into a large 

 artery in the neck (Fig. 86) and if 

 this tube be open at the end, the 

 blood will, of course, flow out under 

 its pressure. But if the tube be 

 placed in a vertical position and 

 mercury be put into it to hold the 

 lood back, the more forcibly the 

 blood presses, the more mercury will 

 be required to hold it back and keep it from rising and 

 coming out. It takes about nine inches of mercury in the 

 tube to prevent the blood from rising. For this reason we 



FIG. 86. SHOWING METHOD 



OP MEASURING BLOOD 

 PRESSURE 



At 6 is a glass tube tied within 

 an artery of some animal; c, a 

 flexible rubber tubejd, a glass 

 tube containing mercury. The 

 greater height of the mercury 

 in the open arm of the tube 

 shows the pressure. 



