Chap. 12 CIRCULATION AND TRANSPORTATION BODY FLUIDS 201 



become mature; in all other vertebrates they are retained. Leucocytes or white 

 cells are of several kinds; none of them has hemoglobin; all have nuclei. In 

 mammals blood also contains colorless bodies called platelets. 



Red Cells. The red cells. (erythrocytes) of mammals (except camel, llama) 

 are biconcave discs (Fig. 12.4). It is estimated that 3000 human red cells 

 set in a line would make a row less than an inch long. The number in a cubic 

 millimeter of blood is calculated as four and a half million for women and five 

 million or more for men. It varies slightly during a 24-hour period, being 

 lowest in the early morning, and increasing through the day. In a healthy 

 person it is increased during exercise, at high altitudes, and with a rise in the 

 temperature of the environment. 



In microscopic preparations and within the living capillaries red cells often 



- BIRDS. 



HUMMING BIRD i PHEASANT 1 PIGEON 



OSTRICH 



Fig. 12.4. Relative sizes of red blood cells, all microscopic, of representative 

 vertebrates. The size of the red cells varies much in different classes of vertebrates; 

 that of the white blood cells, not shown here, is more uniform. Their extremely 

 minute size, coupled with relatively large surface exposure, is a key to the 

 efficiency of the mammalian red blood ceils in their intake and outgo of oxygen. 

 The absence of nuclei in all mature mammalian red cells allows extra room for 

 oxygen. (Courtesy, Guyer: Animal Biology, ed. 3. New York, Harper & Bros., 

 1941.) 



