294 ANIMAL BIOCHEMISTRY 



The iron in hemoglobin can be oxidized to the ferric state, yielding 

 the brown-colored methemoglobin. Methemoglobin, which may arise 

 in vivo as a result of drug or poison action, is incapable of oxygen 

 transport. Carbon monoxide combines with hemoglobin to form 

 carbon monoxide hemoglobin, a complex with a much greater stability 

 than oxyhemoglobin. If a significant portion of the hemoglobin is 

 complexed in this manner, symptoms of anoxia develop, followed by 

 coma and death. The metabolic elimination of the porphyrins is 

 discussed in Chapter 20. 



A hemoglobin, comparable in heme structure but containing a 

 different globin constituent, has been reported in the root nodules 

 of bacteria. Presumably this hemoglobin participates in nitrogen 

 fixation (page 226) although the exact role has not been defined. 



Leucocytes. White blood cells (leucocytes) are normal constituents 

 of blood. In human blood the normal white cell count may fall as 

 low as 5,000/cu. mm., and maximal normal counts of 10,000 are not 

 uncommon. The average white cell count is about 7,000/cu. mm. of 

 blood. 



Under the microscope these cells are present in many characteristic 

 forms. One type consists of small and large lymphocytes, which reach 

 the blood via the lymphatic system. Others are described as large 

 mononuclear, transitional, and polymorphonuclear, depending on the 

 shape and appearance of the cell nuclei. Others are classified according 

 to their staining characteristics. For example, eosinophiles are stained 

 red with eosin, whereas basophiles are stained a blue color with 

 methylene blue. 



The polymorphonuclear leucocytes constitute about 65 to 70 per 

 cent of the white cells of the blood. They possess amoeboid movement 

 and have the ability to engulf and destroy bacteria and foreign sub- 

 stances in blood. This process is known as phagocytosis, and the 

 amoeboid white cells are known as phagocytes. Consequently leu- 

 cocytes function to combat infections. In acute infections the white 

 cell count tends to increase markedly. In acute appendicitis, for 

 example, the white cell count may be as high as 15,000 to 20,000/cu 

 mm. of blood. A white cell count is used routinely to assist the physi- 

 cian and the surgeon in diagnosing this disease and other types of 

 infections. 



Blood platelets. The blood platelets are much smaller than cither 

 the leucocytes or erythrocytes. They vary in size from 1.5 to 3 /x. 

 Their chemical composition is lipoprotein and simple protein in 

 nature. Platelets are extremely unstable and diHicult to isolate as they 

 disintegrate on removal from the circulatory system. Platelets are 



