390 



RESPIRATION AND EXCRETION 



The respiration of cells. 



nearly dependent on the amount of work performed. From 

 twenty to twenty-five ounces is taken in and used by the body 

 every day. Oxygen is constantly taken from the blood by tis- 

 sues in a state of rest and is 

 used up when the body is at 

 work. This is proved by the 

 fact that in a given time a 

 man, when working, gives off 

 more oxygen (in carbon di- 

 oxide) than he takes in during 

 that time. 



While work is being done 

 certain wastes are formed in 

 the cell. Carbon dioxide is 

 released when carbon is burned. But when proteids are bm-ned, 

 another waste product containing nitrogen is formed. This must 

 be passed off from the cells, as it is a poison. Here again the 

 blood and lymph, common carriers, take the waste material to 

 points where it may be excreted or passed out of the body. 



Organs of Excretion. — All the 

 life processes which take place in a 

 living thing result ultimately, in 

 addition to giving off of carbon di- 

 oxide, in the formation of organic 

 wastes within the body. Such or- 

 ganic waste contains nitrogen, and 

 in animals is usually called urea. 

 In man, the skin and kidneys per- 

 form this function, hence they are 

 called the organs of excretion. 



The Human Kidney. -^ The hu- 

 man kidney is about four inches 

 long, two and one half inches wide, 

 and one inch in thickness. Its color 

 is dark red. If the structure of the medulla and cortex (see Fig- 

 ure above) is examined under the compound microscope, you 

 will find these regions to be composed of a vast mmiber of tiny 

 branched and twisted tubules. The outer end of each of these 



Longitudinal section of kidney. 



