RESPIRATION 



157 



so that fresh portions of water are being continually brought 

 into contact with the gills, just as the swaying movements 

 of water plants bring them into contact with fresh areas of 

 water. 



Some fishes contain air sacs which serve two purposes: 

 (i) as a reserve for gas exchanges ; (2) as a float for keeping 

 the animal almost of the same specific gravity as the water. 



The composition of the contained gas varies according to 

 whether rapid changes of volume are required. Those fishes 

 that remain at about the same depth have a gas in their swim 

 bladders which is mainly nitrogen, but those that change 

 rapidly from one depth to another have mainly oxygen 

 instead of nitrogen. The reason for this difference is that 



FIG. 32. Diagram of gill of 

 fish showing how water is 

 brought close to the circula- 

 ting blood. 

 FIG. 31. Drawing of stem of Pota- 



mncy^trm fannatiV rlan^ cTinwincr The blood flows in fine Vessels con- 



mogeton (aquatic plant), showing tained in the fing e r .ii ke processes and by 



air passages in COltex. maintaining a flow of water over the gill 



Copied from " Vegetable Physiology" exchange of gas between the blood and 



J.R. Green (Churchill). water is facilitated. 



nitrogen is soluble to only a small extent in blood, whilst 

 oxygen can be carried in larger quantity. 



The swim bladders of fish that contain large quantities 

 of oxygen are furnished with an oxygen-secreting gland. 

 The problem of secretion of oxygen is of the same nature as 

 other secretions. The increase in concentration of oxygen 

 can be accomplished only as the result of expenditure of 

 energy. Absorption of oxygen takes place from another 

 portion of the swim bladder.* 



Secretion of oxygen in the swim bladder of fish is important 

 because it shows the possibility of secretion of a gas. During 

 normal quiet respiration gas exchange in the lungs of mammals 

 is easily accounted for by the physical process of diffusion. 

 When living at a high altitude where the oxygen tension in 

 * W. N. F. Woodland, Proc. Zool. Soc., 1911, p. 183. 



