RESPIRATION 



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the whole amount of air which the lungs contain. Even after a forced 



expiration the lungs are almost half full ; the air which remains is 



called the residual air. The air 



which is expelled from the lungs 



by a forced expiration, less the 



tidal air, is called the reserve, or 



supplemental, air. These several 



quantities are easily estimated. 



(See Practical Work.) In the 



average individual the total capacity 



of the lungs (with the chest in 



repose) is about one gallon. In 



forced inspirations this capacity may 



be increased about one third, the 



excess being known as the com- 



plemental air (Fig. 43). 



FIG. 43. Diagram illustrating lung 



Internal, or Cell, Respira- capacity, 



tion. -- The oxygen which 



enters the blood in the lungs leaves it in the tissues, passing 

 through the lymph into the cells (Fig. 44). At the same 



Lymph 



FIG. 44. Diagram illustrating internal respiration and its dependence on 

 external respiration. (Modified from Hall.) (See text.) 



time the carbon dioxide which is being formed at the cells 

 passes into the blood. An exchange of gases is thus 

 taking place between the cells and the blood, similar to 



