DIGESTION AND RESPIRATION 535 



collapse all of the alveoli and respiratory passages. Since the inspired 

 air always mixes with a certain amount oi stale air already in the lungs, 

 alveolar air always has a lower oxygen content and a higher content of 

 carbon dioxide than atmospheric air. Alveolar air is also saturated with 

 water vapor. 



Respiratory movements are cyclic and are controlled by inspiratory 

 and expiratory centers (collectively called the respiratory center) in the 

 medulla of the brain. The inspiratory center sends out impulses along 

 the nerves to the inspiratory muscles (neuron ^1, Fig. 26.10 B), and we 

 breathe in. The alveoli fill with air, become stretched, and the resultant 

 sensory impulses traveling to the respiratory center inhibit inspiration 

 {#~}- At the same time, impulses that were initiated in the inspiratory 

 center and took a rather circuitous route within the brain reach the 

 expiratory center {#S), and stimulate it to send impulses out to the ex- 

 piratory muscles (^4). We breathe out, another volley of impulses leaves 

 the inspiratory center, and the breathing cycle begins again. The in- 

 spiratory center tends to be active all the time, ceasing to send out 

 impulses only when it is momentarily inhibited. 



This, in brief, is the basis for our regular breathing, but many other 

 factors can affect the rate and dejjth of respiration. Increased metab- 

 olism during exercise, for example, results in an increased carbon diox- 

 ide content of the blood. This stimulates the respiratory center, and we 

 automatically breathe faster and deeper. The same thing happens when 

 we voluntarily hold our breath. Since the lungs are not being ventilated, 

 carbon dioxide accumulates in the alveolar air and blood, and eventually 

 reaches a level that stimulates the respiratory center and we breathe 

 again involinitarily. One cannot suffocate by holding one's breath. 



By expiring vigorously and frequently, we can reduce the carbon 

 dioxide content of the alveolar air and blood below normal limits, and 

 breathing stops until carbon dioxide accumulates again. The accumula- 

 tion of carbon dioxide in the blood is responsible for initiating breathing 

 in a newborn baby. 



Receptors in the larynx and trachea can also affect respiration. If 

 food inadvertently enters these passages, these receptors are stimulated 

 and a very vigorous expiration, i.e., a cough, results. The cough reflex 

 is one of many safeguards in the body that are activated if something 

 goes wrong with the primary control mechanism, in this case the swal- 

 lowing reflex. 



Questions 



1. How do the teeth of mammals differ from those of lower vertebrates? 



2. \V'hat normally pre\ents food from going down "the wrong way'" when we swallow? 

 What happens if it does start down the larynx? 



3. \Vhat reasons can yon give for the absence of a stomach in ancestral vertebrates? 



4. ^Vonld you expect rennin to be present in the stomach of the young of non-mam- 

 malian vertebrates? 



5. What prevents the wall of the digestive tract from being digested? 



6. How is it possible for herbivorous vertebrates to digest cellulose? 



7. If one were to eat a ham sandwich, where and by what would its various components 



