CHAPTER VI. 

 BLOOD PRESSURE. 



EFFECT OF CHEMICAL SUBSTANCES ON THE BLOOD 

 VESSELS AND HEART. 



The blood stream must be maintained at a certain pressure in 

 order to furnish the tissues a sufficiently rapid supply of oxygen and 

 nutrition as well as to carry away waste products. The heart is 

 directly responsible for the movement of blood, so that any increase 

 in its output will increase the movement. Such an increase may be 

 obtained either by a greater rate or a greater amplitude. 

 Both means are used. 



The blood pressure would tend to fall to zero between the con- 

 tractions of the heart if it were not for the elasticity of the artery 

 walls. With each beat of the heart the arteries are expanded. 

 As soon as the force of the heart is spent, the arteries contract, thus 

 preventing a fall to zero. Fluctuations in pressure are partly 

 prevented by the resistance offered by the capillaries. 



The lowest pressure which is maintained is called the diastolic 

 pressure, because it occurs during the diastole of the heart. The 

 pulse pressure is the difference between the systolic and diastolic 

 pressures. The more elastic the blood vessels, the smaller is the 

 pulse pressure. 



The blood supply to different regions of the body may be in- 

 creased by a greater output, or by a shifting of the blood from one 

 region to another. The latter method is made possible by the 

 presence of vasomotor nerves. Certain nerves cause constriction, 

 while others cause dilatation. Thus the supply to the muscles 

 can be increased by constriction of the vessels in the abdominal 

 organs and dilatation of those in the muscles. 

 Experiment 32. The Measurement of Blood Pressure in 



Man. A sleeve containing a rubber bag, which connects with 



a mercury manometer as well as with a valved rubber bulb, is 



71 



