RESPIRATORY ORGANS. l6/ 



The air is purified by plants. All green plants, when 

 in the light, take up carbonic acid from the air. It is 

 one of their chief foods. From the carbonic acid, they 

 pick out the carbon and use it in making starch and 

 sugar and oils, and other things. The oxygen, they give 

 back to the air. Thus plants not only make food for 

 animals but keep the air fit for them to breathe; while 

 animals by their breathing supply food for plants. 



10. The Air inside the Lungs must be Frequently 

 Changed. If the air inside the lungs be not frequently* 

 replaced by fresh air, it becomes so full of carbonic acid 

 that it can take no more from the blood; and so poor in 

 oxygen that it cannot supply the blood with enough of 

 that gas. Dark-colored venous blood comes to the lungs 

 by the pulmonary artery (Chap. XIII.), containing little 

 oxygen and much carbonic acid. Through the thin walls 

 of the pulmonary capillaries, it gives carbonic acid to, 

 and takes oxygen from, the air inside the lungs, and 

 thus, replenished and purified, is returned to the left side 

 of the heart to be distributed over the body. 



11. The Respiratory Organs are (i) the lungs, in which 

 the blood is exposed to the action of the air ; (2) the 

 air-passages, through which air enters and leaves the 

 lungs; (3) certain muscles (muscles of respiration), and 

 the skeleton of the thorax, which work -together to al- 

 ternately expand and contract the chest, and thus renew 

 the air inside the lungs. 



10. What happens if the air within the lungs is not frequently re- 

 newed ? What sort of blood does the pulmonary artery bring to the 

 lungs ? What does this blood do as it flows through the pulmonary 

 capillaries ? What becomes of it after leaving the lungs ? 



11. Name the respiratory organs. Use of the lungs? Of the air- 

 passages? Of the respiratory muscles and the skeleton of the 

 chest? 



