CHAPTER XIX 



THE ORGANS OF EXCRETION 



371. We have already learned that a double vital proc- 

 ess is continually carried on by the living cells in the 

 tissues of the body. One side of this vital activity is the 

 taking up, from the blood, of oxygen and the nutrient 

 material which the blood receives from the food in the 

 alimentary canal, and it results in growth and repair. 

 The other side of cell activity is the oxidation, or decom- 

 position by burning in the tissues, of worn-out matter, 

 and its return to the blood to be expelled from the body. 

 This removal of waste matter from the blood is called ex- 

 cretion. The waste material from the tissues leaves the body 

 under three principal forms, as carbon dioxide, water, and 

 urea. The lungs, as we have seen, not only supply oxygen 

 to the blood, but also give off' daily a large amount of water 

 and carbon dioxide. Two other organs also have the 

 function of excretion. They are the skin and the kidneys. 

 The skin gives off water and certain salts; the kidneys 

 remove urea and other nitrogenous waste, along with a 

 large amount of water. 



372. The Skin as an Excretory Organ. We have studied 

 the skin as an organ of sensation, and have learned some- 

 thing of its structure and its use as a sense organ and as 

 a protector of the more delicate parts. Now we are to 

 study it as a remover of waste. 



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