182 RESPIRATION 



is arched around the lower part of the heart and lungs. When 

 its muscular portion contracts, the diaphragm flattens and moves 

 downward, while the ribs are elevated. The spaces between the 

 ribs are filled by muscles. When these muscles relax the ribs 

 return to their original position. This reduces the chest cavity 

 from side to side. The diaphragm now returns to its original 

 position and the abdominal walls contract and push the liver and 

 stomach against the diaphragm, which contracts, presses against 

 the lungs, and in so doing, pushes out the walls of the chest 

 cavity, front and back. Thus the chest cavity is increased in size 

 from top to bottom by the contraction of the diaphragm, and 

 from back to front and side to side by the activity of the muscles 

 between the ribs. 



The lungs are as large as the cavity they occupy. When the 

 chest cavity increases in size, the lungs are no longer pressed 

 upon, and, since they are somewhat elastic, they fill the space 

 made by the expanding chest. The air in the lungs now spreads 

 out to fill the space formed by the enlarging of the lungs. Move- 

 ment of air always occurs when there are differences in pressure. 

 As the air spreads out, its density becomes lower. Air rushes in 

 from the outside to equalize the pressure. This is inspiration or 

 the taking in of air. The blood vessels in the air sacs are thus sup- 

 plied with air from which the haemoglobin extracts oxygen and 

 to which it gives up carbon dioxide. 



Expiration is the forcing out of air. It is due to the relaxation 

 of muscles which crowd the lungs into a smaller space. Air in the 

 lungs is then denser than air outside and it is expelled in order to 

 equalize the pressure. This is expiration. The taking in of the 

 air, the exchange of gases in the air sacs, and the giving out of air 

 is breathing or respiration. 



Cell respiration. When the oxygen of the air gets into the blood, 

 it is carried to the cells. It passes from the blood through the 

 walls of the capillaries, through the lymph spaces and into the cells. 



