THE TRACHEA AND LUNGS 



143 



form capillary networks which surround and lie on the walls of 

 the air-cells. These walls of the air-cells are extremely thin, as 

 are also the walls of the capillary vessels, and thus the blood is 

 brought almost in contact with the inspired air. 



It is in these capillaries that the venous blood becomes con- 

 verted into bright arterial blood, by exchanging carbonic acid 

 gas collected in the tissues of all parts of the body for oxygen gas 

 absorbed from the air contained in the air-cells. 



The blood thus changed is collected by small veins, formed 

 by the union of the capillaries ; and these veins unite, forming 

 larger veins, ultimately giving rise to the pulmonary veins, which 

 pour their contents directly into the left auricle of the heart. 



Although we are continually expiring and inspiring air, yet the 



Fig 133. The Pulmonary Circulation. 



i, heart ; 2, right lung ; 3, left lung ; 4, right auricle ; 5, left auricle ; 6, right ventricle ; 7, 

 left ventricle ; 8, pulmonary arteries ; 9, pulmonary veins ; 10, aorta ; n, venje cavse. The 

 arrows indicate the direction of the blood-stream. The shading represents venous blood. 



lungs are never empty in fact, the amount of air which enters and 

 leaves the lungs during ordinary breathing is small compared with 

 that which remains after each expiration. 



The air which passes in and out of the lungs in ordinary quiet 

 breathing is called the tidal air, and amounts to from 20 to 30 

 cubic inches in an adult person. After each expiration^ however, 

 about 200 cubic inches still remain in the lungs, this being termed 

 the stationary air. By making a very deep expiration we can 

 expel about a half of this (the supplemental air), and the lungs 

 then contain about 100 inches of residual air which cannot be 

 expelled. 



