HOW THE ANIMAL LIVES 227 



416. The difference between the artery- blood 

 and vein -blood is shown in the following table : 



Volg. Vols. of 



of carbon 



oxygen dioxid 



From 100 vols. of arterial blood may be obtained . . 20 39 

 " " " "venous " " " " ..8tol2 46 



417. The excess of oxygen in the arterial 

 blood is used up as it passes through the capil- 

 laries, and is replaced by carbon dioxid. The 

 excess of carbon dioxid brought back by the 

 venous blood is thrown out into the air fill- 

 ing the lungs, and is replaced in the blood 

 by the oxygen taken up from the air. The 

 carbon dioxid is made up of one atom of car- 

 bon obtained by the breaking up of the tis- 

 sues or blood elements which contain carbon, 

 and of two atoms of oxygen carried to such 

 tissue or element by the blood. 



418. Breathing, therefore, or the combination 

 of oxygen with carbon to form the carbon di- 

 oxid, really does not take place in the lungs, 

 but in the various parts of the body to which 

 the blood carries the oxygen. 



5c. Amount of air required 



419. The amount of carbon dioxid passed 

 into the blood and exhaled by the lungs is in- 

 creased by exercise, work, sunshine and food ; 

 hence the necessity for more rapid breathing 



