208 



capillaries, in turn, are joined to small 

 veins. More and more veins join, forming 

 larger veins through which the blood 

 flows away from the organ. 



Substances are transported through the 

 arteries and veins very rapidly. Within a 

 few minutes drugs absorbed by the di- 

 gestive system or gases breathed into the 

 lungs can be found in any organ of the 

 body. 



What is blood? Blood consists chiefly 

 of an almost colorless, slightly straw- 

 colored liquid, called plasrim. Plasma is 

 about 90 per cent water. In it are dis- 

 solved the digested foods which have 

 been absorbed in the small intestine and 

 the various wastes which are constantly 

 being added from the working cells. 

 Plasma also contains various types of pro- 

 teins which are of great importance in a 

 number of ways. One of them, for in- 

 stance, helps in blood clotting. It is called 

 fibrinogen (fye-brin'o-jen). Besides all 

 these substances, plasma contains hor- 

 mones (one hormone, secretin, was dis- 

 cussed on page 200), and it carries 

 substances which help us fight disease. 



You can see that plasma is not a simple 

 substance. The make-up of plasma is not 

 always the same; plasma is constantly 

 changing. If the cells in some part of the 

 body are carrying on oxidation at a rapid 

 rate, wastes will be added to the plasma 

 in large amounts. During sleep the 

 amount of waste material present in the 

 transportation system is less. Some hours 

 after a large meal, when the digestion of 

 food is well on its way, the plasma will 

 contain large amounts of substances pro- 

 duced by digestion. If the meal was 

 largely beefsteak the plasma will be rich 



How a Complex Animal Uses Food unit iv 



Platelets 



Nongranular 

 cell body 



Nucleus 



White corpuscles 



Fig. 210 Three kinds of blood cells. How do 

 these cells differ in size and shape? ^Vhat part 

 usually found in a cell is inissing in the red 

 corpuscles? Of what use is each kind of cell? 



in amino acids which come from protein 

 digestion. If the meal was largely starches 

 and sweets the plasma will contain more 

 sugars. When you are exercising, the 

 absorbed food substances are rapidly 

 enteriniT the working cells. You have 

 seen the loading and unloading of a bag- 

 "■auc car at a station. The contents of the 

 car change at every stop; just so with the 

 plasma. Only the blood does not have to 



