Chap. XIII] RESPIRATORY SYSTEM 245 



the air introduced by the first inspiration remains in the lungs, suc- 

 ceeding breaths unfold the lungs more and more, until finally 

 the air-sacs and bronchioles are all opened up and filled with 

 air. The lungs thus once filled with air are never completely 

 emptied again until after death. 



Frequency of respiration. — Each respiratory act in the adult 

 is ordinarily repeated about eighteen times per minute. This 

 rate may be increased by muscular exercise, emotion, etc. Any- 

 thing that affects the heart-beat will have a similar effect on the 

 respirations. The ratio to the pulse is about 1 to 4 in health. 

 Age has a marked influence. The average rate in the newly born 

 infant has been found to be forty-four per minute, and at the age 

 of five years, twenty-six per minute. It is reduced between the 

 ages of fifteen and twenty to the normal standard. 



Respiratory sounds. — The entry and exit of the air are ac- 

 companied by respiratory sounds or murmurs. These murmurs 

 differ as the air passes through the trachea, the larger bronchial 

 tubes, and the bronchioles. They are variously modified in lung 

 disease, and are then often spoken of under the name of " rales." 

 In labored breathing the contraction of the respiratory muscles 

 not usually brought into play, such as the muscles of the throat 

 and nostrils, becomes very marked. 



Effects of respiration upon the blood. — While the blood is 

 passing through the pulmonary capillaries, the following changes 

 take place : (1) it loses carbon dioxide ; (2) it gains oxygen, 

 which combines with the reduced haemoglobin of the red corpuscles 

 and turns it into oxyhsemoglobin, and as a result of this the crim- 

 son color shifts to scarlet ; (3) the temperature is slightly reduced. 



Capacity of the lungs. — As the lungs are not emptied at 

 each expiration, neither are they filled. If filled to their utmost, 

 they can hold a little more than one gallon (4500 c.c.) of air. 

 This total is divided as follows : — 



(1) Tidal. (3) Reserve. 



(2) Complementary. (4) Residual. 



Tidal air is the air introduced with every ordinary inspira 

 tion. 



Complementary air is the excess over the tidal air which may 

 be introduced during a forced inspiration. 



