Fig. 219 In A a vem is bulged at the poiiit 

 where a valve has stopped the backward flow 

 of blood. In B a vem is ctit open through the 

 valve. In which direction does blood flow in 

 this vein? 



PROBLEM 3. How Materials Are Moved to and fro?H Cells 



reason a person whose blood pressure is 

 abnormally high should not engage in 

 strenuous exercise. The pressure may 

 rise so much that some smaller vessel may 

 burst, allowing the blood to escape into 

 surrounding tissues. If this happens in 

 the brain there is a cerebral hemorrhage. 

 As the blood escapes and clots, it causes 

 temporary or permanent paralysis by 

 damaging the delicate brain cells. 



Fainting. It sometimes happens, for a 

 variety of reasons, that the heartbeat is 

 not forceful enough to push the blood 

 uphill into the arteries running into the 

 head. You may have seen a person's face 

 and lips grow pale suddenly. Blood in 

 sufficient amounts is not being sent up 

 into the head; the person loses conscious- 

 ness and loses control of his skeletal 

 muscles; he faints. Frequently he can 

 avoid fainting by holding his head down 

 between his knees or lying flat on his 

 back. Fainting in most cases is not a sign 

 of any special defect. But it should be 

 called to the attention of a physician if 

 it occurs repeatedly. 



An aviator may have a similar experi- 

 ence. When he makes a very fast and 

 sharp turn or pulls out of a fast dive 

 sharply, the blood in his body is pushed 

 toward the outside of the curve. Since 

 this is away from the head, the blood 

 pressure may not be great enough to 

 force blood to the arteries of the head. 

 As soon as the brain cells fail to receive 

 the necessary oxygen unconsciousness 

 occurs. This is the "blackout" pilots talk 

 about. They "see black" as they faint. 

 As soon as the pressure of blood coming 

 from the heart is greater than the force 

 pushing the blood back the aviator re- 

 covers. 



217 



Blood returns by means of veins. The 



finest branches of arteries open into 

 capillaries. Here the spurting motion of 

 the blood is lost. It flows more slowly 

 and smoothly, pushed by the force of the 

 blood behind it in the arteries. From the 

 network of microscopic capillaries which 

 lie in every part of the body, the blood 

 flows into wider vessels, the veins. These 

 unite with one another, forming larger 

 and larger veins, the largest of which 

 empty into the heart. Their walls con- 

 tain some elastic tissue, but since the 

 blood lost its spurting motion in the 

 capillaries it flows smoothly through the 

 veins, forced onward largely by the 

 pressure of the blood behind. 



But the pressure of blood flowing in 

 the capillaries is not always sufficient to 

 push blood uphill. The blood in the veins 

 of the legs, for example, may tend to 

 stop and flow backward. This is pre- 

 vented by valves which are flaps like 



