XXI. THE BLOOD AND ITS CIRCULATION 



Problems. - - To discover the composition arid uses of the dif- 

 ferent parts of the blood. 



To find out the means by whicJi tlie Wood is circulated 

 about the body. 



LABORATORY SUGGESTIONS 



Demonstration. -- Structure of blood, fresh frog's blood and human 

 blood. Drawings. 



Demonstration. --Clotting of blood. 



Demonstration.- -Use of models to demonstrate that the heart is a force 

 pump. 



Demonstration. --Capillary circulation in web of frog's foot or tadpole's 

 tail. Drawing. 



Home or laboratory exercise.- -On relation of exercise on rate of heart 

 beat. 



Function of the Blood. - The chief function of the digestive 

 tract is to change foods to such form that they can be absorbed 

 through the walls of the food tube and become part of the blood. 1 



If we examine under the microscope a drop of blood taken from 

 the frog or man, we find it made up of a fluid called plasma and two 

 kinds of bodies, the so-called red corpuscles and colorless corpuscles, 

 floating in this plasma. 



Composition of Plasma. - The plasma of blood is found to be 

 largely (about 90 per cent) water. It also contains a considerable 

 amount of protein, some sugar, fat, and mineral material. It is, 

 then, the medium which holds the fluid food that has been ab- 

 sorbed from within the intestine. This food is pumped to the body 

 cells where, as work is performed, oxidation takes place and heat 

 is given off as a form of energy. The almost constant temperature 



1 This change is due to the action of certain enzymes upon the nutrients in va- 

 rious foods. But we also find that peptones are changed back again to proteins when 

 once in the blood. This appears to be due to the reversible action of the enzymes 

 acting upon them. (See page 307.) 



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