i8 



Head 



Chest and arms 



Right auricle 



Kidneys 



Legs- 



Left ventricle 



Liver 



Stomach 

 and intestines 



Fig. 220 Diagram of circulatio?? of blood from 

 left ventricle to right auricle. Blood is forced 

 out of the left ventricle through the aorta. 

 Through what large organs does it flow? 

 Branches to some of the smaller organs are not 

 shown here. What kind of blood vessels are 

 shown in black? Does tl^e blood in them con- 

 tain iimch or little oxygen? See Figure 221 for 

 circidation from right ventricle to left auricle. 



How a Complex Aiih/ial Uses Food unit iv 



to move it onward. There is another 

 force that keeps the blood moving on- 

 ward to the heart. When you move 

 about, particularly when you exercise 

 actively, the inner parts of your body 

 press against one another. Muscles, and 

 even many of the internal organs, change 

 size and shape constantly. As they do this 

 they squeeze the veins within them or 

 next to them. When the vein is squeezed 

 the blood moves forward toward the 

 heart since the valves prevent it from 

 going backward. 



Eventually it reaches the auricles and 

 flo\\s into them with a steady flow. But 

 when the auricles are full, the muscles of 

 their walls contract and force the blood 

 into the ventricles below. 



The course of the blood. Imaoine that 

 you are small enough to seat yourself on 

 one of the red corpuscles for a ride 

 around the body. Suppose you started 

 from the left ventricle arid were shot into 

 the large artery known as the aortal (av- 

 or'ta). Soon the aorta branches, one 

 branch leading to the head, another to 

 the arms. At this point you might part 

 company with some of your friends who 

 were riding on other corpuscles. You 

 continue, let us say, down the main ar- 

 tery toward the legs. But immediately 

 you are saying good-by again to more 

 of your friends, some of whom turn off 

 to the stomach, some to the intestine, and 



patch pockets on a coat. The valves oc- 

 cur at regular distances all along the 

 veins. See Figure 2 19. If now you will take 

 time to do Exercise 8, you will learn how 

 to find the location of some of the valves 

 in the veins of your arm or hand. 



Valves can prevent the blood from 

 flowing backward but they have no force 



some to other internal organs. The ar- 

 tery along which you are travelling has 

 become a narrower tube and iww 

 branches equally, one branch leading 

 down each leg. "S'ou happened to go into 

 the left branch and soon find yourself 

 in the left foot, in a very small artery. 

 Suddenh' things look different to you. 



