BLOOD PRESSURE 



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open in the direction of the blood flow. Capillaries are a net- 

 work of very thin-walled vessels through which food, oxygen, and 

 colorless corpuscles pass out to the tissues. 



The pulse and blood pressure. The pulse is caused by the 

 contraction of the ventricle which causes a wave of distention to 

 travel along the blood vessel. This pulse can easily be felt in the 

 larger arteries that are near the surface of the body. As the blood 

 is forced from the heart into the arteries it comes under pressure 

 caused by the resistance given to the flow of blood by the small 

 capillaries. Thus a definite blood pressure is caused, which is 

 seen when we cut an artery. Blood pressure can easily be meas- 

 ured by an instrument called the sphygmomanometer. 



Practical Exercise 5. Visit a physician and have him explain what happens 

 when he takes your blood pressure, and the significance of what he finds. 



Lymph vessels. The lymph is collected from the various tissues 

 of the body by ducts provided, like the veins, with valves. The 

 pressure of the blood within the blood vessels continually forces 

 more plasma into the lymph ; thus a slow current is maintained 

 from the lymph spaces into lymph tubes. On its course the lymph 

 passes through many lymph glands, where impurities appear to 

 be removed. The lymph ultimately passes into a large tube, the 

 thoracic (thS-ras'ik) duct, and empties into the blood stream in the 

 neck region. 



Explain, with reference to your text, why it is that blood flows in one direction in the veins. 



