CHAPTER II 



RESPIRATION 



The air which surrounds us is in the main composed 

 of two gases, oxygen and nitrogen, thoroughly mixed 

 but not chemically combined. Of these the nitrogen is 

 inert while the oxygen is essential to animal life. The 

 organ which enables us to bring this gas in contact with 

 the blood is the lung, while its use in the animal econ- 

 omy is called respiration. But taking oxygen into the 

 lungs, would be of no service if the process stopped 

 there. It is, therefore, carried by the blood into all 

 parts of the body where the oxygen is used and inju- 

 rious matter taken up and removed. If two gases be 

 placed one on either side of a wet membrane, like parch- 

 ment, they will mix with each other by a process called 

 osmosis. If, therefore, a liquid, like blood, containing 

 oxygen be brought in contact with tissues containing 

 carbon dioxide, the two gases would interchange even if 

 there were no chemical activity to promote the change. 

 Blood going to all parts of the body carries oxygen in 

 combination with hemoglobin. When in contact with 

 live tissue this oxygen is given up to the tissue and car- 

 bon dioxide is taken up and carried by the venous blood 

 to the lungs where, in the cells of those organs, a re-ex- 

 change is effected, the carbon dioxide being given up 

 and oxygen, derived from the inspired air, takes its 

 place. The last exchange is called the external and the 

 first the internal respiration. 



The organs of respiration are the nose, pharynx, 

 larynx, trachea and lungs. Except the last, and in all 



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