CHAPTER XXXI 

 THE MECHANISM OF RESPIRATION 



IN the unicellular organism, oxygen is taken in from the 

 surrounding medium, and the C0 2 given out through the cell sur- 

 face. In the moie complex organisms, a special respiratory apparatus 

 becomes evolved, and two kinds of respiration are distinguished: 

 external respiration, by which oxygen is taken from the surrounding 

 medium into the circulating transport fluid the blood and the CO 2 

 given off from this fluid to the medium ; internal respiration, by which 

 a gaseous interchange takes place between the body fluids and the 

 various body cells, oxygen being taken to the cells, and C0 2 removed 

 from them. Internal respiration is probably the same process in all 

 forms of animals. External respiration differs with the stage of 

 development, and also with the habitat of the animal. The lower 

 forms of invertebrates usually breathe through the skin; the higher 

 forms, when living in the water, by a specially developed system of 

 gills; when land dwellers, by a special system of branching tubes 

 known as " tracheae," which carry the air diiect to the blood -spaces 

 surrounding the individual cells. 



In low temperatures, the frog can breathe by its skin alone. In 

 man, 1-5 per cent, of the respiratory exchange is reckoned to take 

 place through the skin, and somewhat more on sweating. A small 

 amount of respiratory exchange takes place through air which is 

 swallowed: certain fishes breathe by this method. 



Among the vertebrates, fishes have a special system of gills, by 

 which the gaseous interchange between the surrounding water and 

 the blood is effected. 



Amphibians in the larval stage e.g., the tadpole also possess 

 gills; but in the adult animal these are replaced by lungs. In the 

 higher vertebrates, after birth, external respiration is always effected 

 by means of specially developed lungs. During development, how- 

 ever as, for example, in the human foetus traces of the remains of 

 the gill apparatus can still be seen. 



In the higher organisms, the circulatory mechanism suffices for the 

 transport of the gases to and from the tissues, and various devices 

 have been adopted to facilitate the interchange between the outer 

 medium and the respiratory apparatus. In fishes, the water is taken 

 in through the mouth, passed over the gills, and out by the gill-slits; 

 in many amphibia, the air is swallowed into the lungs. In birds, 

 during rest, the movements of the ribs suffice to draw air through the 

 lungs and in and out of the five large air-sacs. The lungs are not 



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