HUMAN STRUCTURE AND LIFE- ACTIVITIES 507 



EXCRETION 



432. Purpose of Excretion. The products of oxidation 

 in the cells of the human body are of no further use, and when 

 accumulated may be injurious. Hence special organs have 

 as their work the elimination of excretions (1) from the 

 blood and lymph which absorbs them from the cells, and 

 (2) from the body. Most important of the excretions are, 

 as in the case of all other organisms we have studied, carbon 

 dioxide (C02), water (H 2 0), and nitrogenous excretions. The 

 carbon dioxide is chiefly excreted by the lungs, the water 

 by the kidneys (with some unnecessary help by the skin), 

 the nitrogenous excretions chiefly by the kidneys. 



433. Excretion of Carbon Dioxide. Venous blood enter- 

 ing the lungs contains about 46 cc. of carbon dioxide in 

 100 cc. of blood, and the arterial blood leaving the lungs 

 has only about 40 cc. Thus it appears that approximately 

 one-eighth of the contained carbon dioxide is excreted as 

 blood flows through the capillaries in the lungs. 



The chief difference between arterial and venous blood is 

 that the arterial has twice as much oxygen and seven-eighths 

 as much carbon dioxide. The small amount of nitrogen 

 dissolved in the blood is always the same (1 to 2 per cent) , for 

 free nitrogen takes no part in the activities of living cells in 

 animals. 



Evidently it is incorrect to state that " venous blood is 

 purified in the lungs " and that " arterial blood is pure." 

 The loss of only one-eighth of the carbon dioxide is not making 

 " pure." We should not say that we have made muddy 

 water " pure " if only one-eighth of the mud is extracted. 

 The words "pure" and " purify " should never be used in 

 connection with respiration of blood in the lungs. It is 

 easy to remember that blood doubles its oxygen and loses 

 about one-eighth of its carbon dioxide in changing from 

 venous to arterial blood in the lungs. 



