RESPIRATORY CHAMBERS 143 



answer to that question has been given by Dr J. S. 

 Haldane of Oxford University. He discovered that the 

 respiratory chambers or sacs of the lungs are laden with 

 air which is always charged with the same proportion 

 of C0 2 — namely, 5*6 volumes of C0 2 to every 100 

 volumes of air. He observed that if the proportion of 

 C0 2 becomes higher, then the breathing is quickened, 

 thus washing out more C0 2 from the respiratory chambers 

 and replacing it with air. If the proportion becomes less 

 than normal, then the breathing is slowed until the usual 

 proportion is restored. It is now quite well known why 

 the rate of breathing alters as the proportion of carbon 

 dioxide in the air sacs is altered. In health, arterial blood 

 is slightly alkaline in reaction ; if it contains a charge of 

 C0 2 it becomes acid in reaction, and if such blood is 

 supplied to the medullary nerve-centres which control 

 the muscles of respiration, then these centres are excited 

 and send out messages which set the muscles of respira- 

 tion to work more vigorously until the blood regains its 

 normal quality. Not only are the respiratory nerve 

 centres disturbed by an excess of C0 2 in the blood supplied 

 to them ; they are also thrown into an equally excitable 

 state if there should be a deficiency of oxygen. The rate 

 of our breathing is automatically controlled by the per- 

 centage of oxygen and carbon dioxide in the arterial blood 

 — the two substances connected with combustion. The 

 amount of these substances in the blood depends on their 

 proportion in the air filling the respiratory chambers of 

 the lungs. The more we work, the greater is the com- 

 bustion in our bodies. The greater the combustion, the 

 more rapidly must we renew the air in the respiratory 

 chambers by breathing. In this manner a very simple 

 and yet very effective mechanism has been invented by 

 Nature to keep the respiratory bellows working at a rate 

 which exactly meets the needs of the human machine. 

 We can now see why Nature has arranged the mouths of 

 the respiratory chambers so that they become automa- 

 tically closed at a certain stage of emptying. At least 

 one-third of the charge of air is retained. The automatic 



