182 RESPIRATION 



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

 size of the lungs is in direct proportion to the size of the chest 

 cavity. When the chest cavity increases in size, the lungs are 

 no longer pressed upon, and, since they are somewhat elastic, 

 the air within them pressing on their inner surfaces, causes them 

 to expand. As the air in the lungs spreads out to fill the spaces 

 formed by the enlarging of the lungs, its density becomes lower. 

 Since movement of air always occurs when there are differences 

 in density or pressure, the air from outside the body rushes in 

 and thus equalizes the pressure. This is inspiration. 



Expiration is the forcing out of air from the lungs. It is due 

 to the relaxation of muscles which crowd the lungs into a smaller 

 space. The air in the lungs is then under greater pressure, and 

 consequently denser than the air outside, and it is expelled in order 

 to equalize the pressure. The taking in of air, an exchange of 

 gases between the air sacs and blood vessels, and the giving out of 

 air is breathing ov respiration. The exchange of gases is mainly 

 a withdraw^al of oxygen by the haemoglobin and the replacement 

 of carbon dioxide by the blood plasma. (Diagram p. 184.) 



Changes in the size and the shape of the chest cavity and in the 

 volume of air in the lungs are brought about by the muscular 

 activity of the diaphragm and the muscles attached to the ribs. 

 The normal position of the diaphragm is arched into the chest 

 cavity. When the diaphragm contracts, it flattens and moves 

 downward. This increases the size of the chest cavity from top 

 to bottom. Ribs are attached to the breast bone in front and to 

 the spinal column in the back. Muscles are attached to the ribs 

 and when they contract the ribs are elevated. Thus the walls 

 of the chest cavity are pushed out from back to front and from 

 side to side by the activity of the rib muscles. 



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. 



