PROBLEM 3 How Are Materials Moved to and from 



Our Body Cells? 



The transportation system. You have 

 learned that in the human ahmentary 

 canal there is large scale digestion of 

 food and that digestion produces mate- 

 rials that may be assimilated or oxidized 

 in the cells. But there are bilhons of cells 

 in the body, most of them far removed 

 from the alimentary canal where diges- 

 tion takes place. Digested foods are trans- 

 ported to the cells by a transportation 

 system called the circulatory systeju. 

 There are really two systems. One sys- 

 tem is composed of the heart and blood 

 vessels through which blood moves. The 

 other consists of tubes called lymphatics 

 which carry lymph. Let us study the cir- 

 culation of the blood first. 



Blood circulates in a system of two 

 connected sets of tubes. It is pumped to 

 all parts of the body by the heart through 

 one set, the arteries. It flows back to the 

 heart through another set, the veins. 



How food enters the blood system. Di- 

 gested foods in the small intestine are 

 absorbed by the villi. Each villus contains 

 tubes of two kinds. There is a network 

 of tiny blood vessels which are part of 

 the blood system; and there are tubes 

 called lacteals which are part of the 

 lymphatic system. The blood vessels in 

 the villi are microscopic with very thin 

 walls. Such tiny blood vessels with ex- 



tremely thin walls are known as capilla- 

 ries (cap'ill-a-rees). Digested foods easily 

 enter them and become part of the blood. 



Capillaries in all organs. Just as there 

 is a small network of capillaries in the 

 villi, there are netM'orks of capillaries in 

 every organ, such as the brain, the in- 

 ternal organs, the muscles of the whole 

 body, and the skin. You cannot see these 

 capillaries in your body because they are 

 microscopic but by doing Exercise i 

 you can get a very good idea of how 

 they must look. They are so tiny and 

 branch so widely that they are spread 

 through every part of the body. Every 

 cell is more or less closely in contact 

 with capillaries. 



The digested foods diffuse out through 

 these capillaries which lie among all the 

 cells of the body; thus the food sub- 

 stances reach the living cells. Every or- 

 dinary cell engages in many activities, 

 but the activities are on so small a scale 

 that you are not aware of what is going 

 on. One of these activities is oxidation. 

 Not only food but oxygen as well dif- 

 fuses out of the capillaries into the neigh- 

 boring cells. As a result of oxidation, 

 energy is released and new substances are 

 fonned. Some of these are harmful to 

 protoplasm; at best, they are useless. 

 They are the waste products. The wastes 



