396 



TEXTBOOK OF ZOOLOGY 

 Average Characteristics of Human Blood Cells 



(Reproduced by peri«is.«ion from Textbook of Zooloay by Storer, copyrighted 

 1943, by McGraw-Hill Book Co., Inc.) 



of the heartbeat for an average adult man at rest is about 72 times 

 per minute. The contraction phase of the heartbeat is called the 

 systole and the relaxation phase is the diastole. It has been estimated 

 that an average circuit of the circulation of blood in man can be com- 

 pleted in twenty-three seconds, with about two seconds of this time 

 being spent in capillaries. 



The Excretory System. — The excretory system of vertebrates con- 

 sists of kidneys, excretory ducts, and often a urinary bladder. The 

 kidneys serve to remove from the blood, waste nitrogen products 

 and excess salts in solution as well as to dispose of excess water. 

 In simpler vertebrates there is a pronephros type of kidney as well 



