HUMAN STRUCTURE AND LIFE-ACTIVITIES 505 



clothing is worn, the diaphragm is but little used, and ex- 

 pansion of the chest-cavity is due chiefly to movements of 

 the ribs. The ideal breathing movements combine to use 

 both ribs and diaphragm. Note effect of forced breathing, 

 or after rapid exercise. 



429. Changes in Breathed Air. Analysis of air which 

 has been taken into the lungs and then expired shows that 

 it has lost oxygen. This means that the blood circulating 

 in the capillaries of the lungs has absorbed some oxygen from 

 the air in the air-chambers around which the blood-capillaries 

 are arranged. 



The average adult person can expel from his lungs after 

 the deepest possible inspiration about 230 cubic inches of 

 air. In ordinary breathing a man takes in about 30 cubic 

 inches, and expels about the same amount. An additional 

 100 (total 130) cubic inches can be taken in by forced in- 

 spiration (" a long breath "), and this much plus about 

 100 not usually expired (total 230) can be expelled by forced 

 expiration, as stated above. 



After the deepest possible expiration of 230 cubic inches of 

 air, about 100 cubic inches of air remain in the lungs. It is 

 obvious that under ordinary breathing conditions the amount 

 of air inspired (30 cubic inches) is not sufficient to fill the 

 lungs, which contain 130 cubic inches, for it is not quite one- 

 third of the amount left in the lungs. The fact is that the 

 100 cubic inches of air usually left in the lungs is mixed and 

 diluted with the 30 of fresh air. 



430. Absorption of Oxygen by Blood in Lungs. The 

 blood-supply to the lungs is venous, from the right side of 

 the heart, which in turn receives it from all the organs except 

 the lungs. This blood passes through the capillaries of 

 the lungs and leaves by way of the pulmonar}' veins to the 

 left side of the heart, thence to all the organs except the lungs. 

 On the way through the lungs the blood is changed from venous 

 to arterial, by losing part of the carbon dioxide ( 433) and 



