PROBLEM 3. How Materials Are Moved to and jrom Cells 



207 



Fig. 208 Part of the web of a frog's foot ?nag- 

 nified 75 tiines. The irregular dark spots are 

 coloring matter in the skin. The very faint, nar- 

 row vessels are capillaries. What are the wider 

 vessels? In what ways is the blood chatiged 

 while it is in the capillaries? (hugh spencer) 



formed in oxidation diffuse through the 

 capillary wall into the blood. As you 

 continue to learn how the human body 

 performs its life activities you will dis- 

 cover that there are still other substances 

 which enter and leave the thin-walled 

 capillaries in all the organs of the body. 

 Long distance transportation to and 

 from the organs. The tiny, thin-walled 

 capillaries connect the longer and wider 

 arteries and veins. Transportation from 

 one region to another is through the 

 wider tubes. The walls of arteries and 

 veins are much thicker than the walls of 

 capillaries. Blood flows through arteries 

 and veins over long distances to and from 

 the organs. Within each organ the artery 

 branches into smaller and smaller arteries. 

 The smallest of these arteries connect 

 with capillaries within the organ. The 



Heart 



Fig. 209 The heart and some of the large veins 

 (dark) and arteries (light) of the main circida- 

 tory system. The heart is the organ which 

 pimips the blood to all parts of the body 

 through the arteries. The blood flows back to 

 the heart through the veins. Connectijig the ar- 

 teries and veins are the capillaries (see Fig. 208). 

 Every cell in the body lies near one of these 

 microscopic tubes. How are digested foods and 

 oxygen obtained by the living cells, and how 

 are waste products carried away? The ly?ft- 

 phatic system is not shown in this diagram. 



