VITAL CAPACITY 



259 



A separate chapter has been devoted to the mechanism whereby 

 the oxygen is brought from outside into the port. Briefly, by 

 muscular movements air is drawn through filtering and warming 

 appliances into the harbour, and after a very short interval is 

 expelled from the harbour into the outer air. 



In the average resting man, somewhat over 500 c.c. of air come 

 into the respiratory chambers at each ordinary quiet inspiration 

 and somewhat less than this amount is expelled at each expiration. 

 One may say, in round numbers, that the tidal air of the average 

 adult is about 500 c.c. The tidal air at various ages and weights 

 has been determined and is given below : 



TABLE XLI. 



If a very deep inspiration is taken, more than 500 c.c. can be 

 sucked into the lungs. This extra quantity, which varies with 

 the " build " and expertness of the subject but which usually is 

 about three times the tidal volume, is called the comple mental air. 

 By a forced expiration after a normal inspiration the reserve air 

 may be expelled from the lungs. The volume so expelled varies 

 very much with practice. Some people can only breathe out an 

 additional 500 to 700 c.c., while singers, physical training experts 

 and others who practise abdomino-thoracic breathing,' may 

 register a volume of 1500 to 2500 c.c. These three quantities 

 together give the vital capacity of an individual, i.e. the amount 

 of air that a person can expire after a deep inspiration. It is not 

 possible completely to empty the lungs. As we shall see later, 

 a mechanism exists in the air vesicles which prevents their total 

 collapse. They retain about a litre of air the residual air. 



To summarise, taking average figures : 



j Tidal air 500 c.c. 



Vital capacity Complemental air 1500 c.c. 



( Reserve air 1500 c.c. 



Residual air 



Volume of fully distended respiratory apparatus 



3500 



- 1000 c.c. 



- 4500 c.c. 



