MECHANISM AND CHEMISTRY OF RESPIRATION 199 



breaths several times a day, filling and emptying the lungs as 

 completely as possible. 



CAUSE OF RESPIRATORY MOVEMENTS 



The Cause of Respiratory Movements. None of these facts, 

 however, tell us much about the real purpose of respiration 

 or the actual cause of the respiratory movements; they 

 only describe how the movements are produced and 

 regulated. 



The respiratory movements are started in the respiratory 

 center which sends regular, rhythmical messages down the 

 cord to the breathing muscles. What excites this center into 

 such regular activity? In answer to this question an interest- 

 ing experiment may be described. By a complicated method 

 it is possible to send through the brain, and thus through the 

 respiratory center, blood different from that which goes to the 

 rest of the body. If blood containing only a small amount 

 of oxygen is sent through it, the center begins to send out 

 messages very rapidly to the breathing muscles, even though 

 the rest of the body is receiving very pure blood. On the 

 other hand, if the blood sent through the respiratory center 

 contains an unusually large proportion of oxygen, the breath- 

 ing messages are sent out more slowly than usual. If blood 

 with an extremely large amount of oxygen be used, breathing 

 messages become very slow no matter what kind of blood the 

 rest of the body receives. 



The logical conclusion is that the center in the brain, which 

 controls breathing, is influenced by the condition of the 

 blood which flows through it. When the body is doing more 

 work than usual, more oxygen is needed, and more carbon 

 dioxid is being formed. The blood rapidly loses the oxygen 

 and absorbs carbon dioxid; this impure blood affects the nerve 

 cells of the medulla, causing more rapid breathing. It is the 

 condition of the blood in the respiratory center that ordinarily 

 determines the rate of respiratory movements. 



