156 CIRCULATORY SYSTEM 



usually be distinguished from veins and capillaries by the presence 

 of considerable muscular tissue in their walls. Early anatomists, 

 when dissecting bodies, found these tubes empty and thought 

 that they were supposed to carry air and therefore called them 

 arteries (air-tubes) . Even the smaller arteries have muscular walls 

 and can contract and relax. The aorta and its branches are full of 

 blood all the time. When the left ventricle contracts, the blood 

 cannot move forward into the comparatively narrow arteries fast 

 enough to make room for the new supply sent out by the ventricle. 

 But, due to the elasticity of its walls, the aorta can expand and 

 receive the incoming blood. The impulse of the blood sent into the 

 arteries by the ventricular contraction causes a wave of distention 

 to travel along the blood vessel. This wave of distention is called 

 the pulse. The pulse is the passive stretching and contracting of 

 the elastic tissue. If an increased amount of blood is sent to a 

 particular part of the body, at any time, the arteries can always 

 accommodate it because of the elasticity and muscularity of their 

 walls. The pulse may be felt wherever an artery comes near the 

 surface of the body. Most arteries are deeply embedded in other 

 tissues so that the pulse is not easily noted. Arteries run near 

 the surface in the wrists, neck, and temples. Probably the most 

 convenient place for taking a pulse is in the wrist. 



Problem. Study of the pulse rate. 



I. Place the middle finger of the right hand about two inches from the ball 

 of the thumb of the left hand, thus locating a pulse. Count the number of 

 beats felt per minute. This is called taking the pulse rate. 



II. Run up and down stairs two or three times. Again take the pulse rate. 



A. What effect has exercise on the pulse rate? 



B. Keeping in mind the fact that the walls of the heart and arteries 

 have large deposits of muscle and elastic fibers, give the reason for the 

 effect of exercise on the pulse rate. 



C. Discuss the beneficial effects of exercise on the arteries. 



D. If you get insufficient exercise, what effect would it eventually 

 have on the heart and arteries ? 



