PROBLEM 3. 



Superior 



vena 



cava 



Hoiv Materials Are Moved to and from Cells 



Aorta Right aurick 



Pulmonary 

 artery 



Right and left Veins 

 ventricles 



Right ventric 



Fig. 215 (above) The heart cut open. Note 

 the four chambers. From which chambers and 

 through which tubes does blood leave? 



Fig. 214 (left) The heart. This is the pump 

 that keeps the blood in constant motion. 



be easier to carry; and the other proteins 

 can be used for other purposes. 



The blood is in constant motion. The 

 plasma with its blood cells travels to the 

 farthest regions of your body and reaches 

 every living cell. In organisms like us that 

 walk upright blood travels long distances 

 directly uphill. The blood is in constant 

 rapid motion; a drop of blood may make 

 the rounds of the body in less than half 

 a minute. How is it done? 



Blood is moved in the simplest way; 

 it is pushed. If a liquid is put into a bag 

 and the bag squeezed, the liquid will 

 squirt out of the bag through any open- 

 ing. If all openings but one are closed 

 and the bag is squeezed hard, the liquid 

 will squirt out with force. The heart is 

 so constructed that this happens every 

 time it "beats." The walls of the heart 

 are made of powerful muscles which by 

 contracting do the squeezing themselves. 

 It is so easy to obtain a beef heart which 

 is constructed like yours that you will 



want to do Exercises 4 and 5 to learn 

 about your heart. 



The heart of a human being consists 

 of four parts. Two of these, the upper 

 ones, push the blood into the lower ones; 

 they do this gently. They are the right 

 and left auricles (ori-k'ls), thin- walled 

 chambers which collapse when not filled 

 with blood. The other two chambers lie 

 below the auricles. They are called ven- 

 tricles (ven'tri-k'ls), the right and left 

 ventricle. They are larger chambers with 

 much thicker walls. By contracting they 

 squirt the blood into big vessels carrying 

 blood away from the heart. They con- 

 tract with great force. The right and left 

 sides of the heart are completely sepa- 

 rated from one another by a thick and 

 solid wall. Blood cannot pass directly 

 from one side to the other. It is just 

 as though there were two distinct hearts. 



The outer walls of the two auricles and 

 the two ventricles are continuous so that 

 from the outside the heart looks as 



