INSPIRATION 381 



the lungs; hence the lungs are not rigid, but elastic. The 

 air sacs of the lungs number many millions. Their sur- 

 face in an adult has been estimated to be over 2,000 

 square feet. This is the floor area of a room 20 feet wide 

 and 100 feet long. 



383. Exchange of Gases in the Lungs. A large ar- 

 tery (the pulmonary artery) carries dark-red blood from 

 the heart's right ventricle to the lungs. This blood is 

 spread out through the network of capillaries that is in 

 the air sacs, and loses its carbon dioxide. At the same 

 time it takes up oxygen, and becomes bright red. Then 

 it is carried, by the pulmonary veins, to the heart's left 

 auricle. This pumps it into the left ventricle, by which 

 it is forced, through the aorta, to the body. 



The air that enters the lungs contains 21% oxygen, 78% nitrogen, 

 and 1% argon. As it leaves the lungs, it contains about 16% oxygen 

 and nearly 5% of carbon dioxide, water vapor, and other impurities. 

 The loss of oxygen is thus balanced by the gain in carbon dioxide. The 

 amounts of nitrogen and argon are not changed. About 1.2 pounds of 

 water and 1.5 pounds of carbon dioxide are exhaled from the lungs daily. 

 As the air exhaled is warm, and as the water is in the form of vapor, 

 the lungs aid the skin in removing heat from the body (cf. 395). 



384. Inspiration. The external respiration (breath- 

 ing) consists of 2 acts: (1) inspiration, or "breathing in"; 

 and (2) expiration, or "breathing out." The entrance of 

 air into the lungs (inspiration) is brought about by the 

 enlarging of the chest. This permits the air in the lungs 

 to expand, and makes its density and pressure less than 

 that of the air outside. The outer air then rushes into the 

 lungs as it would into a vacuum (cf. 6 and 42). We 

 do not "suck" air into the lungs any more than we suck 



