CIRCULATION OF THE NUTRIENTS 115 



blood vessels have to be stretched in order to accommodate 

 the additional blood that is forced into them. Hence, 

 when the ventricles begin to relax, the blood tends to rush 

 back into these chambers from the arteries. To prevent 

 this, valves are placed at the opening of each of the two 

 arteries that lead from the right and left hearts (Fig. 33), 

 Each valve is shaped like a watch pocket. The three open 

 outward from the heart, but as soon as the ventricles begin 

 to relax, the blood fills up the pockets, and the three valves, 

 by meeting in the middle of each artery, keep the blood from 

 returning to the ventricles (Fig. 33, A). 



162. Position of the capillaries. As we trace the arteries 

 farther and farther from the heart, we see that they divide 

 and subdivide until very small branches are formed. That 

 these fine branches are still arteries is proved by the fact 

 that elastic and muscular tissues are present in their walls. 

 Finally, however, these tiny blood vessels become continuous 

 with still smaller tubes, the capillaries. So numerous are 

 the capillaries that one cannot push the point of a needle 

 for any considerable distance into any organ of the body 

 without piercing a number of them. These smallest of 

 blood vessels communicate freely with one another and 

 form a complicated network of tubes that bring blood close 

 to all cells of the body. 



163. Importance of the capillaries. If the blood were 

 kept constantly within a system of tubes like the arteries, 

 it would be entirely unable to help in the nutrition of the 

 body because osmosis would be impossible. Each cell of 

 the body must take from the blood the nutrients it needs 

 for its special work; likewise it must give off to the blood 

 the wastes it has formed by oxidation. It is through the 

 thin-walled capillaries that all these exchanges of material? 



