156 The Vertebrate Organ Systems 



system. Normally both contract and relax at a regular rate. Unlike 

 heart muscle, however, this contraction is not an inherent property of the 

 muscle, for it ceases if the nerves are cut. Adult human beings when 

 at rest have a respiratory rate of from 14 to 20 per minute. This rate 

 is increased during muscular exertion, and varies with the age and con- 

 dition of health of the individual. 



During exercise, there is a marked increase in the respiratory 

 rate. This is due to the increase of the carbon dioxide concentration 

 in the blood. This acts as a stimulus to the respiratory center of the 

 brain, which in turn causes an increase in the rate. Experimentally it 

 has been shown beyond question that the rate of respiration is correlated 

 with the amount of carbon dioxide in the blood. Without carbon 

 dioxide, respiration would cease for it is the stimulus. Respiration 

 can be voluntarily stopped for short periods, but as carbon dioxide 

 accumulates in the blood, respiration automatically begins. 



Capacity of the Lungs. — Immediately after birth, the lungs are 

 filled with air. After that time, they are never completely emptied. 

 The amount of air which the lungs can hold varies from individual to 

 individual and is known as the vital capacity. 



During normal respirations, the amount of air exchanged is about 

 500 c.c. This is the tidal air. It is possible by means of a forced in- 

 spiration to increase the amount of inspired air by about 1,600 c.c. 

 This has been called complemental air. A forced expiration after a 

 normal inspiration reveals that at least 1,600 c.c. more of air is present 

 in the lungs. This is the supplemental air. The total of these three 

 forms the vital capacity. 



Complemental air 1,600 c.c. 



Tidal air 500 c.c. 



Supplemental air 1,600 c.c. 



Vital capacity 3,700 c.c. 



Within the alveoli there is always some air which cannot be forced 

 out. This is about 1,000 c.c, and is known as the residual air. Of 

 course, the molecules of this residual air are not always the same as 

 there is some mixing with the inspired air. 



Second Wind. — During prolonged exercise, the feeling of distress 

 and breathlessness gradually disappear. This is due to the so-called 

 second tvind, which is actually an adjustment on the part of the cir- 

 culatory and respiratory systems to the increased demands of the active 



