CHAPTER XVI 

 THE BLOOD AND ITS CIRCULATION 



Problems : To discover the composition and uses of the different 

 parts of the blood. 



To find out the means by which the blood is circulated through the 

 body. 



To study the ductless glands and their secretions. 



Laboratory Suggestions 



Laboratory demonstration. Structure of blood, fresh frog's blood and human 

 blood. Drawings. 



Laboratory demonstration. Clotting of blood. 



Laboratory demonstration. Use of models to demonstrate that the heart 

 is a force pump. 



Laboratory demonstration. Capillary circulation in the web of a frog's foot 

 or a tadpole's tail. Drawing. 



Home or laboratory exercise. The relation of exercise to the rate of heart beat. 



Composition of the Blood. — We learned in the last chapter that 

 the chief function of the digestive organs is to change foods so 

 that they can pass into the blood. The chemical composition 

 of the blood is very complex and varies in different parts of the 

 body. The fluid part is the plasma, which consists of water 

 (about 90 per cent) and the various organic food substances (about 

 10 per cent) digested sugars, fats, and amino-acids, mineral salts, 

 and numerous other substances, among which are enzymes and 

 hormones. The blood also holds three kinds of bodies, the red 

 corpuscles, the colorless corpuscles, and the blood plates. 



The blood is the great go-between connecting the outside world 

 and the body cells. It carries gases to and from the cells; it 

 carries the chemical messengers, the mysterious hormones, which 

 play so important a part in our bodily activity ; it carries the 

 an'tibodies of various kinds which fight diseases ; it carries the 

 red and the colorless corpuscles, which are so necessary to our very 



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