EXCHANGE OF AIR IN THE LUNGS 319 



5. EXCHANGE OF AIR IN THE LUNGS 



The volume of air taken in with an ordinary inspiratory effort is estimated 

 for the adult man at about 500 c.c. With a frequency of 16 per minute this 

 would give a ventilation volume (breath volume, Rosenthal) of 8,000 c.c. = 

 8 liters, per minute. 



According to Gregor, the average breath volume of children in the first 

 month amounts to 1,300 c.c. per minute, in the twelfth month 3,000, and between 

 the second and thirteenth years it varies between 4,000 and 5,000 c.c. 



After an inhalation of the average volume, a considerable quantity of air 

 can still be taken into the lungs, and after an ordinary expiration a consider- 

 able quantity can still be expelled from the lungs. But if we make the most 

 extreme expiratory effort with the assistance of all the expiratory muscles, 

 there remains in the lungs a certain quantity of air which, so long as the 

 thorax is uninjured, can never be expelled. 



This air left over is called the residual air. Attempts have been made, 

 by various methods which cannot be described here, to determine its amount, 



FIG. 127. The number of respirations per minute in persons of different ages, after Quetelet. 



and, if we neglect the values which are obviously incorrect, it has been found 

 to vary from 500 to 1,600 c.c. We shall probably make no great mistake if 

 we estimate it for the healthy adult man at 1,000 c.c. in round numbers. 



The reason why the residual air cannot be expelled from the lungs is simply 

 that when entirely collapsed the lungs inclose a much smaller space than does 

 the thorax contracted to its smallest capacity. Since the lungs are pressed 

 against the thoracic wall by the air inclosed within them, their volume cannot 

 of course be diminished beyond the volume of the chest itself. When, however, 

 the thoracic wall is opened, and the air pressure inside and outside the lungs is 

 thereby equalized, they collapse in virtue of their own elasticity and drive out 

 the contained air. 



