270 ANIMAL BIOLOGY 



while the carbon dioxide will diffuse into it until the two 

 gases are evenly distributed on either side of the mem- 

 branous wall. 



The ability of the blood to take up large quantities of 

 oxygen is dependent upon the fact that the haemoglobin 

 of the red corpuscles forms a chemical combination with 

 this element. When supplied with oxygen the blood 

 turns red as may be demonstrated by shaking up bluish 

 venous blood with oxygen or even with air, when it takes 

 on a bright reddish color. Similarly red blood becomes 

 bluish when shaken up in an atmosphere of carbon dioxide. 

 As we have already seen, the blood returning from the 

 lungs where it becomes exposed to the air is red in color 

 and remains red in the arteries that supply the organs of 

 the body. After it has passed through the capillaries 

 and is returned through the veins it acquires a bluish tint. 

 As chemical tests show, it has lost a considerable part 

 of its oxygen and has received a larger amount of carbon 

 dioxide. The constant need for oxygen to keep up the life 

 of the tissue and the necessity for the removal of carbon 

 dioxide, which is produced where vital changes are going 

 on, make it so very important that the activity of breath- 

 ing should go on unchecked. 



While we speak of the lungs as the organs of respiration 

 it must be remembered that respiration occurs in all the 

 cells of the body. They all take oxygen from the blood 

 and give off carbon dioxide. This process is often distin- 

 guished as internal or tissue respiration. But as the blood 

 circulates through the lungs it absorbs oxygen from the air 

 and gives off its carbon dioxide, a process which we may 

 designate as external respiration. The blood acts as a 

 carrier between the tissues and the respiratory organs, a 

 function, as we have seen, for which it is admirably adapted. 

 Oxidation is essentially a process of burning, and the con- 



