380 



CIRCULATION AND RESPIRATION 



(2) To remove carbon dioxide from the blood. The 

 circulation of the blood brings about oxidation at the 

 cells, and also carries the carbon dioxide to the lungs. 

 The oxidation at the cells is internal respiration. The 

 exchange of gases between the air and the lungs is external 

 respiration, or breathing. The chief organs of external 

 respiration are the lungs, and the air passages which 

 connect the lungs with the air. To these we must add the 

 muscles and bones of the chest, which aid in breathing. 



382. The Lungs. The lungs (Fig. 287) are two spongy 

 organs that hang in the thorax, and are surrounded by 



two sacs of connec- 

 tive tissue. The sacs 

 are called pleurae 

 (singular, pleura). 

 The heart cavity is 

 between them. The 

 lungs are the meet- 

 ing place of air and 

 blood; hence we find 

 them to be a mass of 

 air passages and air 

 cells on the one hand, 

 and of blood vessels 

 on the other. Con- 

 nective tissue holds 

 these together. The lungs are lined with mucous mem- 

 brane, just as the other air passages are (cf. 358). The 

 entrance of air into the lungs, and its passage out of the 

 lungs, depend upon the expansion and compression of 



FIG. 287. 



The Lungs and Their Surroundings, Including the 

 Diaphragm. 



