THE BREATHING SYSTEM 



477 



the same way, at the cellular level. The 

 methods employed by different animals to 

 bring oxygen to the cells are highly varied 

 and become extremely complex in large 

 animals. Such a system may be compared 

 to the ventilating system of a large build- 

 ing. The larger and more complex the 

 building, the more intricate the system be- 

 comes, and each new expansion of the edi- 

 fice means the addition of more powerful 

 blowers and an ever increasing number of 

 ducts. A point might even be reached when 

 it would be impossible, or at least imprac- 

 tical, to attempt to provide ventilation to all 

 parts of the building. In animals, likewise, 

 the problems become more complex with 

 increasing size, which has doubtless been 

 one of the factors in limiting the size of 

 animals. Otherwise, they might grow for- 

 ever. In earlier chapters, reference has been 

 made to the breathing systems of the va- 

 rious animal groups, and here they are 

 brought together in a comparative manner 

 (Fig^lS-l). 



Unicellular animals such as amoeba have 

 no special devices for breathing. They take 

 oxygen through the outer covering and lose 

 carbon dioxide the same way, by the simple 

 process of diffusion. The same is true of 

 hydra. In planaria, however, the body is 

 much more complex, and tissues between 

 the body wall and the gastrovascular 

 cavity are remote from the surface, so 

 that diffusion is much less efficient. This 

 definitely limits the size of the animal. 

 Modifications had to be evolved, therefore, 

 before any increase in bulk could occur. In 

 the earthworm this was accomplished by 

 the introduction of a circulatory system 

 which carries oxygen from the skin (the 

 breathing organ) to all cells of the body 

 (Fig. 10-17). Insects solved the problem 

 very differently. They devised a unique 

 system of tubes (trachea) which carries 

 oxygen directly to each cell (see p. 226). 

 In many higher forms, such as larger Crus- 

 tacea, mollusks, and the lower vertebrates, 

 special breathing organs in the form of 



gills have evolved; while in the higher 

 vertebrates, beginning in the Amphibia, 

 lungs have developed. Gills and lungs are 

 intricate structures designed to provide a 

 tremendous surface area which makes pos- 

 sible rapid gas exchange. Gills are designed 

 to function in water, lungs in air. They are 

 essentially alike in having thin-walled sur- 

 faces which are highly vascularized by cap- 

 illaries in order that the blood can come 

 close to the oxygen-containing outer en- 

 vironment. For a more detailed account, we 

 will use the breathing system of man. 



HUMAN LUNGS 



The lungs of man, like those of all verte- 

 brates, arise in the embryo from an out- 

 pocketing of the floor of the pharynx. When 

 fully formed, they fill the entire thoracic 

 cavity. The complete breathing system con- 

 sists of the nasal chambers, larynx, trachea, 

 bronchi, bronchioles, and alveoli (Fig. 

 18-2). The nostrils are the normal portals 

 of entry for air, and they open into tlie 

 spacious nasal chambers which are espe- 

 cially adapted for warming the incoming 

 air. The surface area is greatly increased by 

 sheets of bony tissue, the turbinates, that 

 hang down into the nasal passages. These 

 are covered with a layer of mucous epithe- 

 lium that is kept constantly wet by the 

 mucus-secreting glands located in this 

 layer of tissue. It is also highly vascular- 

 ized, giving warmth to the entire nasal 

 chamber. The result is that as air passes 

 through these passages it is not only 

 warmed to approximately body tempera- 

 tures but is also humidified to some extent. 



Another important feature is the provi- 

 sion made to filter particulate matter out 

 of the incoming air. The nose is lined with 

 hair which strains out the larger particles, 

 and smaller bits which pass the hair are 

 caught in the mucus secretion and eventu- 

 ally discharged with it. Proof of the effi- 

 ciency of this mechanism is noted by ob- 

 serving the color of the nasal secretions 



