PROBLEM 3. Hoiv Materials Are Moved to and fro7Ji Cells 



Fig. 216 What layers do 

 you find in the walls of ar- 

 teries and veins? Which have 

 thicker walls? Hoiv do cap- 

 illaries differ from arteries 

 and veins? 



Outside connective tissue 

 jscular 



lastic 



Cell nucleus 



Artery 



Arteries help move the blood. The ven- 

 tricles contract so forcibly that the blood 

 is squirted well along the artery. When 

 a large artery is cut \'ou can see the 

 blood coming out in spurts. Do you know 

 what first aid procedure to use when an 

 artery is cut? Various procedures may 

 be used: pressure at certain points or a 

 tourniquet are most common. It would 

 be \\t\\ if all of us joined a first aid class 

 and learned how to stop bleeding. 



The arteries which are attached to 

 each of the two actively pumping ven- 

 tricles have walls which contain a large 

 amount of elastic tissue. This is true of 

 all arteries, even those that are at some 

 distance from the heart. As each rush of 

 blood strikes these elastic walls, the ar- 

 tery stretches and at once comes together 

 again, as any elastic substance tends to do. 

 In this way the blood is squeezed within 

 the artery and helped along its course. 

 You can feel the walls of an artery pul- 

 sating (beating) whenever you put your 

 finger over an artery that lies near the 

 surface. In most parts of your body the 

 arteries are buried deep within the tis- 

 sues, but in your temples, in your wrists, 

 and in some other places they are close 

 to the skin. Here you can feel them 

 stretching with each squirt of blood. 



Smooth 

 lining membrane 



Capillary 



■i ^ 



t^^r^r^t^*^':;^ 



Fig. 217 A piece of a cat's intestine showing 

 hrajiching arteries and veins. William Harvey 

 saw small arteries and veins, but he could not 

 see that they were coijnected. How are they 

 co7inected? (clay adams co.) 



This stretching of the artery is called 

 the pulse. Each pulse beat is caused by 

 the rush of blood sent along the artery 

 with each contraction or pump of the 

 heart. Thus, counting your pulse is a 

 convenient way of counting your heart- 

 beats. Whenever your heart beats faster 

 you can notice this difference in your 

 pulse. Try Exercise 7. 



A closer look at arteries. The arteries 

 which arise in the heart soon branch so 

 that the blood goes in several directions. 



