NECESSITY FOR CIRCULATION 



149 



another, four different groups of blood are now recognized. Under 

 ordinary conditions the body can renew blood as fast as it is needed. 

 However, in certain diseases and in cases of severe hemorrhages 

 it is necessary to introduce a quantity of blood into the veins of 

 the patients. In such cases, blood of a similar group must be 

 transfused, or results may be fatal through the dissolving of the 

 corpuscles, and a consequent release of a foreign haemoglobin. 

 Therefore, doctors always test a donor's blood to make sure it is 

 of the same group as the patient's before making a transfusion. 



Necessity for circulation. Each cell in the body has work to do. 

 This work is performed through energy released in the cell. We 

 have already learned that glandular cells use chemical energy, 

 muscular cells use mechanical energy, and nerve cells need nervous 

 energy. All of these forms of energy are obtained from the food. 

 When the food is oxidized in the body, the energy stored in the 

 food is released. Each cell must receive a suppl}^ of oxygen and 

 food in order to carry on the oxi- 

 dation process with its accom- 

 panying release of energy. When 

 the food is oxidized in the cells, 

 carbon dioxide, water, and ni- 

 trogenous wastes are formed, 

 and must be removed. The 

 blood carries the oxygen and 

 food to the cells and removes 

 the wastes from them. Since 

 the white corpuscles combat 

 germs, the blood must carry a 

 continuous supply of these to 

 whatever part of the body that 

 needs them. At the same time other protective chemical sub- 

 stances, antibodies, must be carried to the places needing them. 

 As food is oxidized in various tissues, heat is released. A great 



Wastes formed during oxidation reach the 

 blood by means of the process of osmosis. 



