XXVI. THE BLOOD AND ITS CIRCULATION 



Problem XL Till. To study tlic composition of the blood. 

 {Laboratory Manual, Prob. XLI'III.) 



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 wahs of the food tube and become part of the blood. "^ 

 [By means of a system of closed tubes, this fluid tissue circulates 

 to all parts of the body, equalizing the body temperature by de- 

 positing its burden of food in places where it is most needed and 

 where it will be used, either in the repair and building of tissues 

 or for oxidation within the cells of the body to release energy. J 



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_corg2isdes and c olorless cory vsdss, 

 floating in this plasma. 



Composition of Plasma. — The. plasma ofblood (when chemically 

 examined in man) is found to be largely (about 90 per cent) water. 

 It also contains a considerable amount of proteid, some sugar, 

 fat, and mineral material. It is, then, the medium which holds the 

 fluid food (or at least part of it) that has been absorbed froni" 

 within the intestine. The almost constant temperature of the 

 body is also due, as we shall see, to the blood which brings to 

 the surface of the body much of the heat given off by oxidation of 

 food in the muscles and glands within. When the blood returns 

 from the tissues where the food is oxidized, the plasma brings back 

 with it to the lun'gs the carbon dioxide liberated from the tissues of 

 the body where oxidation has taken place. Blood returning from 

 the tissues of the body has from 45 to 50 c.c. of carbon dioxide 



' This change is due to the action of certain enzymes upon the nutrients in various 

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

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

 acting upon them. (See page 72.) 



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