88 



GENERAL SCIENCE 



The weight of the oxygen will be : 



1.43 X 21 =30 grams 

 99.3 + 30 = 129.3 grams, the weight of 100 liters of air. 



This result agrees with that obtained by actually weigh- 

 ing the air. All four results indicate that air is simply 

 a mixture of these two gases. 



Respiration. Respiration serves two purposes : that 

 of bringing oxygen to the blood and removing carbon 

 dioxide from the blood. As the blood exchanges food 

 and oxygen for the wastes of the tissues, it gradually 

 becomes laden with these waste products which must 

 be removed by the different organs of excretion. Some 



of them are removed by the 

 kidneys, some by the skin, 

 while the carbon dioxide is 

 removed by the lungs. The 

 chief organs of breathing or 

 respiration are the lungs and 

 air passages. 



The Lungs. - - The lungs 

 are two pinkish gray organs 

 of light, spongy appearance 

 (Figure 88). They are lo- 

 cated in the chest cavity. 

 Each lung is surrounded 

 with two layers of an elastic, serous membrane, called 

 the pleura. One layer closely covers the lung, while the 

 other is attached to the wall of the chest in such a way as 

 to form a closed sac. As the lungs change in size these 

 two layers glide on each other with little friction. Pleu- 

 risy is an inflammation of this membrane due to the 

 cessation of the lubricating serum. 



FIG. 88. The Lungs. 



