220 



How a Complex A?iinml Uses Food unit iv 



blood cells and by the plasma in large 

 amounts ever since you were close to the 

 working cells in the foot, diffuses out of 

 the capillary into the lung. Riding on a 

 bright red corpuscle you go from the 

 capillary into a small vein lying among 

 the lung tissues, then into a larger and 

 larger vein (the pulmonary vein). This 

 vein connects with the left auricle. Hav- 

 ing just gone through a second set of 

 capillaries you are again on the left side 

 of the heart. One contraction of the 

 auricle sends you back to your starting 

 point, the left ventricle. All your friends 

 riding on different corpuscles have shared 

 one experience with you; they have all 

 been through a lung capillary. The only 

 difference is that instead of having gone 

 through a capillary in the foot they have 

 gone through a capillary in some other 

 part of the body. You have all been 

 through two loops of blood vessels and 

 by doing this you have made one com- 

 plete circuit of the blood circulatory 

 system. 



If studying this paragraph took you 

 ten minutes, the auricles of your heart 

 filled and emptied themselves into the 

 ventricles about 700 times while you were 

 reading. Since the contraction of the au- 

 ricles is immediately followed by a con- 

 traction of the ventricles your ventricles 

 pumped blood into the arteries the same 

 number of times in this interval. To test 

 your understanding of this paragraph do 

 Exercise 9. 



What prevents backward flow of blood 

 in the heart? When you studied the beef 

 heart you probably found an answer to 

 this question. In a normal heart all the 

 blood in the auricles is pushed into the 

 ventricles; none of it can return from 



.Artery 



Veins 



Left heart 



B 



Fig. 222 /w A the valve between the left auricle 

 and ventricle is open. In B the left ventricle has 

 co7itracted. Why is blood not forced back into 

 the left auricle? Where is it being forced by 

 this contraction? 



the ventricles to the empty auricles be- 

 cause there are flaps of membrane called 

 valves which act as doors. When the 

 ventricles contract, the valves completely 

 shut off each ventricle from the auricle 

 above it. See Figure 222. With the con- 

 traction of the ventricles the blood, 

 therefore, enters the arteries. And none 

 of it has a chance to leak back into the 

 ventricles because there are pocket valves 

 at the mouth of each artery. Thev are 

 like the valves found inside the veins. 

 When blood starts to flow backward 

 toward the heart it catches in the pockets 

 which open up and block the way back. 

 If the edges of any of the valves do not 

 close tightly some blood "leaks" back. 



Body cells are surrounded by a liquid. 

 Many of the cells in the body tissues 

 seem to be very closely packed together. 

 They are not. Tissue cells are always 

 surrounded by a liquid. The liquid is 

 called lymph. Lymph is mostly plasma 

 which has diffused from the capillaries. 

 It is considerably diluted with water. 

 You have seen it; it is the liquid which 

 fills blisters. Lymph contains white cor- 



