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body in case of infection. The blood also contains substances, 

 antibodies, which help combat any disease germs entering the body. 



The blood circulates through the body by means of a network 

 of tubes and is controlled by the heart. Imagine a pump so 

 built that it is self-regulating, so strong that it works day and 

 night without rest, so powerful that it lifts several tons of weight 

 the height of the body every day, year in and year out. Such is 

 the human heart. Although the two sides of the heart are sepa- 

 rate and distinct from each other, yet every drop of blood that 

 passes through the right side of the heart also passes through the 

 left side. It requires from twenty to thirty seconds for the blood 

 to make a complete circuit from the ventricle back again to the 

 starting point. This means that the entire volume of blood in 

 the human body passes through the various organs of the body 

 three or four thousand times a day. 



One of the uses of the blood in its capacity as a carrier is to 

 transport certain chemical activating substances known as hor- 

 mones. There are a good many such substances, the chief of which 

 are manufactured by certain glands known collectively as the 

 endocrines. These structures, of which the thyroid gland is a 

 well-known example, have no ducts or connections with the food 

 tube or other organs. Consequently their secretions can get out 

 only through the medium of the blood stream. The blood dis- 

 tributes the hormones to the body cells, where they cause very 

 great changes to take place through their chemical actions. 



Most of us have had the experience of chopping down a tree, of 

 digging a deep hole, or of lifting a heavy rock. In a very short 

 time we notice that we breathe more quickly and deeply, that we 

 get hot and perspire, and that after a time we become fatigued. 

 What does this sequence of events mean? Evidently we can go 

 back to our old analogy of the engine. To do more work we make 

 a hotter fire, to get a hotter fire we increase the amount of oxygen 

 that gets to the fire by regulating the draft. And we know, too, 

 that if we are to keep up the fire, we must remove the ashes and 

 other wastes frequently. A very similar condition exists in the 

 human body. We have learned that plants and animals need 

 oxygen in order to release energy, just as coal is burned to give 



