102 RESPIRATION 



oxygen which these aquatic animals require is dissolved 

 in the circumambient fluid. If removed from water they 

 cannot live, for they cannot take up oxygen from the atmo- 

 sphere. But this distinction must not be pushed too far, for 

 in vertebrates the inner tissue of the lungs is moist. This 

 moisture takes up the oxygen of the air and transmits it to 

 the blood vessels. Air-breathing animals have about 21 per 

 cent, of the atmosphere to draw on for oxygen, but a litre of 

 average sea-water, under normal conditions, absorbs from 

 the dry atmosphere only 6-44 c.c. of oxygen. To counter- 

 balance the smaller proportion of oxygen available to aquatic 

 animals, there is the extreme solubility of carbon dioxide in 

 water. In consequence of this the carbon dioxide is removed 

 very quickly from aquatic animals, especially from those that 

 breathe partly through the skin, as the frog. This rapid 

 removal of carbon dioxide may not appear of any great 

 assistance to the animal, but carbon dioxide is a waste product, 

 and, like the ashes in a furnace, will extinguish the flame if not 

 removed. Carbon dioxide is, however, not entirely a useless 

 product; it is of the greatest im.portance in controlling the 

 rate of oxidation or combustion of the tissues. When carbon 

 dioxide dissolves in water and in the body fluids, it forms 

 a very weak acid; of course, the acid becomes weaker and 

 weaker the less carbon dioxide there is in solution. Now 

 supposing we take violent exercise, much carbon dioxide is 

 produced which dissolves in the blood and renders it slightly 

 more acid ; this increase in acidity lowers the rate at which 

 oxidation proceeds. On the rate at which oxidation proceeds 

 depends our supply of energy, and if we can get rid of the 

 carbon dioxide as fast as it is made we can keep our energy 

 supply constant. Land animals breathing by lungs have 

 to pant to rid themselves of carbon dioxide ; water breathing 

 animals need to make no such exertion, since in them the carbon 

 dioxide diffuses into the surrounding medium so quickly that 

 little or no carbon dioxide is found in the body fluids. The 

 rate of oxidation being controlled by the carbon dioxide and 

 not by the amount (tension) of oxygen, we see that aquatic 

 animals lose nothing by having less oxygen available for 

 breathing. 



