BREATHING 



173 



that in the air sacs, and the proportion of oxygen in the 

 blood is less than that in the air sacs. 



A diffusion of the gases takes place. 



The blood, by this act of breathing, gains oxygen and 

 loses carbon dioxide. 



The air sacs lose oxygen 

 and gain carbon dioxide. 



234. Respiration in all 

 of the Tissues. The blood 

 thus freighted with oxy- 

 gen travels to the left side 

 of the heart, is pumped 

 out through the aorta, and 

 whirled away to the tissues FIG IIQ BLACKBOARD SKETCH . 

 in every part of the body. Diagram showing Capillary Network of 



The tissues are most eager the Air Sacs and Origin of the Pul- 

 tO Combine with the life- monary Veins. 



giving oxygen and to give A > S 1 \ branch of P ulmo "f y ***y ; * twi * 



& J & of pulmonary artery ; C, capillary network 



around the walls of the air sacs; A branches 

 ^ ne t wor k converging to form the veinlets 

 of the pulmonary veins. 



Up Carbon dioxide tO the 

 Klnnrl 



The oxygen given off 

 to the tissues by the arterial blood in the capillaries may 

 not be used to produce oxidation at once, but may be stored 

 up for future use in muscular and other tissues. 



During severe exercise the amount of carbon dioxide 

 breathed out may often exceed the whole amount of oxy- 

 gen taken in by the lungs during the time of action. In 

 other words, during severe exercise the muscles may use 

 up the oxygen which has been stored in their tissues during 

 periods of rest. 



235. Impurities in the Air. There are many things 

 which may make the air we breathe unwholesome. The 



