262 ELEMENTARY BIOLOGY 



Every cell of the human body, like the cell of the ameba, 

 gets oxygen and gives off carbon dioxid by osmosis. But the 

 body as a whole can keep going only on condition that oxygen 

 is brought to every cell. 



This involves ( i ) a special set of organs for receiving oxygen 

 from the outside — the air passages of the nose and throat ; 

 (2) a special absorbing area closely packed into a relatively 

 small space — the lining of the lungs ; (3) a conducting system 

 that distributes the oxygen and gathers up the carbon dioxid 

 — ^the blood, with the red corpuscles; and (4) a mechanism for 

 alternately filling and emptying the bags containing the absorbing 

 surface — consisting of bones, muscles, and nerves. 



These several distinct organs and tissues act as a unit ; 

 there is some connection, that is, between the amount of 

 oxygen used and the activity of the breathing system. 



315. Excretion. In all the organisms that we have been 

 considering, energesis involves the formation of other sub- 

 stances besides carbon dioxid. But as some of these substances 

 are injurious to protoplasm, their removal is necessary for the 

 continuous life of the cells. In the ameba the wastes simply 

 eliminate themselves by osmosis ; in the many-celled organisms, 

 getting waste out of one cell would simply mean passing it on 

 to another. 



In the daisy, wastes are accumulated in flower and root and 

 in some stem cells, away from the live, active cells. 



In the human body we find that the conducting system (the 

 blood and lymph) absorbs the wastes from the active cells, and 

 that the wastes are then removed from the body by means of 

 special organs (the kidneys, with their connected tubes and 

 bladder, and the sweat glands). 



Again, there is unity in the behavior of these special organs 

 — their activity is related to the activity of the body as a whole, 

 and especially to the circulatory system. 



316. Correlations within the organism. From every point of 

 view that we have considered, the organisms are alike in that 



