Chapter 25 

 BIOLOGY OF MAN 



I. GENERAL ORGANIZATION OF THE HUMAN BODY 



All human beings arise embryologically from a single cell (zygote) 

 which is the result of the fertilization of an ovum (egg) by a male sperm. 

 This zygote and all succeeding cells divide by mitosis to produce the 

 organs, tissues, and cells of which the body is composed. When a child 

 is born its body already is composed of approximately 26 trillion cells. 

 It is suggested that the reader review the discussions of cells and tissues in 

 previous chapters as well as the embryologic development of a human 

 being. 



In spite of the fact that there are so many cells in the human body, 

 they are not all alike. In fact, early in the development of the embryo 

 various cells are set aside (differentiated) to form the future organs and 

 tissues. The human body is composed of the following systems of organs 

 with their functions briefliv stated. 



Integumentary (Skin) System. — Protection, support, heat regulation, 

 absorption, excretion, stimuli reception (Fig. 228). 



Skeletal System. — Support, protection, posture, motion, locomotion, 

 manufacture of blood corpuscles (by bone marrow), transmission of 

 sound waves (ear bones) (Figs. 229 and 250). 



Muscular System. — Locomotion, movements of parts of the body or 

 organs, as stomach, heart, intestines, etc. (Figs. 232 and 233). 



Digestive System. — Ingestion, digestion, and absorption of foods (Fig. 

 236). 



Circulatory System. — Transportation of foods, wastes, heat, oxygen, 

 carbon dioxide, and various secretions (Figs. 237 to 242). 



Respiratory System. — Furnish oxygen and eliminate carbon dioxide 

 and other waste products (Fig. 244). 



Excretor)^ System. — Secretion and elimination of waste products of 

 cell and tissue metabolism (Fig. 245). 



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