148 



BLOOD AND ITS IMPORTANCE 



spleen through mitotic division. When an infection occurs in the 

 body, they multiply in great numbers and later disintegrate or 

 probably escape from the blood upon mucous surfaces, especially 



_ in the intestine. When 



oK^^en 



ox^gtat^ 



^^^ 



plcc^ima: 



The cells take oxygen carried by the red corpuscles 

 and use it to burn food for the release of energy. 



white corpuscles are de- 

 stroyed by bacteria, the 

 dead corpuscles are called 

 pus. An abnormal in- 

 crease of white corpuscles 

 in the blood usually indi- 

 cates the seriousness of 

 the infection and the 

 amount of resistance of- 

 fered by the body. 

 Blood clottmg. Blood platelets are tiny bits of protoplasm 

 found in blood. They are small cells, but lack a nucleus. Their 

 origin is probably similar to the red corpuscles. They are con- 

 cerned with blood clotting which takes place when blood is exposed 

 to air because of wounds or hemorrhages. Blood platelets dis- 

 integrate and give rise to a substance which plays an important 

 part in the change of the liquid protein, fibrinogen, always present 

 in blood, to a solid form, strands of fibrin. Calcium, also present 

 in blood, is involved in the precipitation of the fibrinogen. The 

 strands of the fibrin entangle the blood cells and form a jellylike 

 blood clot. Bleeding may thus be stopped. The body then re- 

 pairs, by building new cells, the blood vessels from which the blood 

 is flowing. People whose blood does not clot in the normal length 

 of time or who bleed profusely even from the slightest wounds are 

 called bleeders. The disease is haemophilia and it is usually 

 hereditary. 



Blood grouping. The composition of human blood is constant 

 with the exception of the oxj^gen and carbon dioxide content. 

 But since the serum of one person may be injurious to that of 



