CHAPTER 9 

 THE RESPIRATORY SYSTEM 



Introduction. Living protoplasm, that is to say a living organism, 

 burns slowly and continually. When oxidation ceases, life ceases also. 

 Galen in the second century saw the similarity between respiration and 

 burning. But it was many centuries before Lavoisier (1771-1780) proved 

 its chemical nature. Breathing is but a subordinate part of respiration. 

 Respiration is the process of gaseous exchange which occurs in a living 

 body through the oxidation of carbon compounds. This exchange involves 

 an intake of oxygen and an outgo of carbon dioxide. The process requires 

 uncombined oxygen, which forms one fifth of the air. Aquatic organisms 

 obtain their oxygen from air dissolved in water. 



Two kinds of respiration may be distinguished, external and internal. 

 In external respiration, animals make use either of a moist skin or of 

 specialized respiratory organs such as lungs and gills in which blood 

 capillaries are brought into intimate relation with moist membranes. 

 Under these conditions, intake of oxygen goes on in accordance with the 

 law of diffusion of gases separated by semipermeable membranes. In inter- 

 nal respiration, in accordance with the same law, gaseous exchange takes 

 place within all the tissues of the body which are bathed with blood or 

 lymph. Cells draw on the oxygen in these just as a burning match gets 

 its oxygen from the air. The living cell, however, unUke the match, is 

 the master of the oxidative process and not its servant. 



The necessity for two kinds of respiratory organs, one adapted to 

 aquatic and the other to land and aerial life, has produced in chordates two 

 distinct but possibly not entirely independent respiratory systems to 

 complicate evolutionary history. These are the pharyngeal gills of the 

 lower and the lungs of the higher classes. Chordates have not inherited 

 their respiratory system from their invertebrate forbears, but have 

 invented new ones of their own. 



Fortunately for the land vertebrates, their fish ancestors were already 

 prepared for the transition from water to land life before the event occurred. 

 By a change of function and some modifications of structure and relation, 

 the bilobed air bladder of the crossopterygian fishes was made to serve as 

 a lung. Furthermore the advantage of nasal passages in air breathing 

 was probably already anticipated by the fish ancestors of amphibians. 

 This assumption seems justified by the fact that some fishes, such as the 



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