332 



RESPIRATION AND EXCRETION 



In other words, there is a loss between 4 and 5 per cent oxygen, 

 and nearly a corresponding gain in carbon dioxide, in expired air. 

 There are also some other organic substances present. 



Cell Respiration. — It has been shown, in the case of very 

 simple animals, such as the amceba, that when oxidation takes 

 place in a cell, work results from this oxidation. The oxygen 

 taken into the lungs is not used there, but is carried by the blood 

 to such parts of the body as need oxygen to oxidize food mate- 

 rials in the cells. Since 

 work is done in the cells 

 of the body, food and oxy- 

 gen are therefore required. 

 The quantity of oxygen 

 used by the body is nearly 

 dependent on the amount 

 of work performed. Oxy- 

 gen is constantly taken 

 from the blood by tissues 

 in a state of rest and is 

 used up when the body is 

 at work. This is suggested 

 by the fact that in a given 

 time a man, when working, gives off more oxygen (in carbon 

 dioxide) than he takes in during that time. 



While work is being done certain wastes are formed in the cell. 

 Carbon dioxide is given off when carbon is burned. But when 

 proteins are burned, another waste product containing nitrogen 

 is formed. This must be passed off from the cells, as it is a poison. 

 Here again the lymph and blood, the common carriers, take the 

 waste material to points where it may be excreted or passed out of 

 the body. 



The Mechanics of Respiration. The Pleura. — The lungs are 

 covered with a thin elastic membrane, the pleura. This forms a 

 bag in which the lungs are hung. Between the walls of the bag 

 and the lungs is a space filled with lymph. By this means 

 the lungs are prevented from rubbing against the walls of the 

 chest. 



The respiration of cells. 



